mt t rn m' " -^ _^ ^„^ _^ ^^ (!l0mrfl Hmrmitj Jibt;M{ .f.:^.a.; 2236 o.in.an^ ^^24 031 714 938 The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031714938 THE INTERNATIONAL SCIENTIFIC SERIES. VOLUME XIV. THE INTERNATIONAL SCIENTIFIC SERIES. Works already PuUished. I. THE FORMS OF WATER IN RAIN AND RIVERS, ICE AND GLACIERS. By J. Tyndall, LL. D., F. R. S. With 26 Illustrations. Price, $1.50. II. PHYSICS AND POLITICS; or. Thoughts on the Application of THE Principles of "Natural Selection" and "Inheritance" to Political SociETy. By Walter Bagehot. Price, $1.50. III. FOODS. By Dr. Edward Smith. Illustrated. Price, $1.75. IV. MIND AND BODY: the Theories of their Relations. By Alexander Bain, LL. D. Price, $1.50. V. THE STUDY OF SOCIOLOGY. By Herbert Spencer. Price, $1.50. Vi. THE NEW CHEMISTRY. By Professor Josiah P. Cooke, of Har- vard University. Illustrated. Price, $2.00. VIL ON THE CONSERVATION OF ENERGY. By Professor Balfour Stewart. Fourteen Engravings. Price, $1.50. VIII. ANIMAL LOCOMOTION; or. Walking, Smimming, and Flying. By Dr. J. B. Pettigrew, M. D., F. R. S. 119 lUustiations. Price, $i.75. IX. RESPONSIBILITY IN MENTAL DISEASE. By Dr. Henry Maudsley. Price,, $i.so. X. THE SCIENCE OF LAW. By Professor Sheldon Amos. Price, $i.75. XI. ANIMAL MECHANISM; or. Aerial and- Terrestrial Locomo- tion. By C. J. Marey, Professor of the College of France, Member of the Academy of Medicine, Paris. 117 Engravings. Price, $1.75. XII. HISTORY OF THE CONFLICT BETWEEN RELIGION AND SCIENCE. By John W. Draper, M.D., LL.D. Price, $1.75. XIII. THE DOCTRINE OF DESCENT AND DARWINISM. By Oscar Schmidt, Professor in the University of Strasburg. Price, $1.50. XIV. THE CHEMISTRY OF LIGHT AND PHOj:OGRAPHY ; w its Application to Art, Science, and Industry. By Dr. Hermann VoGEL. One Hundred Illustrations. {In press.) XV. FUNGI; their Nature, Influence, and Uses. By M. C. Cooke, M. A., LL. D. Edited by Rev. M. J. Berkeley, M. A., F. L. S. With 109 Illustrations. {In press.) XVI. OPTICS. By Professor Lommel, University of Erlangen. {In press.) Plate I. FroJiiis/'iece. irood'dmy /-r Photograph of the Moon, (after RuthiiRFOkd's original negative.: Seep. 193. THE INTERNATIONAL SCIENTIFIC SERIES. THE CHEMISTRY OF LIGHT PHOTOGRAPHY. BT ^jjzJiuuW- Db. HERMANN VOGEL, A ■ FBOFESSOB IN THE BOTAL INDUSTRIAL ACADEMY OF BERLIN. WITH ONE BUNBRED tLLUSIBATIONS, NEW YORK: D. APPLETON AND COMPANY, 549 AND 551 BROADWAY. 1875. PEEFACE. Among the splendid scientific inventions of this century, two are specially prominent — Photography and Spectrum Analysis. Both belong to the province of Optics, and at the same time of Chemistry. While Spectrum Analysis has, down to the present time, remaitied almost exclusively in the hands of the learned, Photography passed immediately into practical life, spread over almost every branch of human effort and knowledge, and now there is scarcely a single field in the universe of visible phenomena where its productive influence is not felt. It brings before us faithful pictures of remote regions, of strange forms of stratification, of fauna, and of flora ; it fixes the transient appearances of solar eclipses ; it is of great utiUty to the astronomer and geographer ; it registers the movements of the barometer and thermometer; it has found an alliance with porce- lain painting, with lithography, metal and book typo- graphy; it makes the noblest works of art accessible ■Vl PEEFACE. to those of slender means. It may thus be compared to the art of printing, which confers the greatest benefit by multiplying the production of thought, for it conveys an analogous advantage by fixing and multiplying phenomena. But it does more than this. A new science has been called into being by Photography, the Chemis- try of Light; it has given new conclusions respecting the operations of the vibrating ether of light. It is true that these services, rendered by Photography to art and science, are only appreciated by the few. Men of science have in great measure neglected this branch after the first enthusiasm excited by Daguerre's inven- tion had evaporated ; it is only cursorily that physical and chemical matters are treated on in manuals of Photography. Taking this into consideration, it has seemed expedient to the Author to give a popular view of Photography and the Chemisti^ of Light, showing their important bearing on science, art, and industry. The Publisher has met the Author in the readiest manner, not only by providing numerous woodcuts to explain the text, but by obtaining specimens of the latest discoveries in Photography, at a considerable cost. So that as the tables annexed give a view of what is achieved by Photography in connection with typography, he trusts the work may meet a friendly reception. THE AUTHOE. Berlin, January, 1874. TABLE OF CONTENTS. CnAITBB 1. Development of our Photo-Chemical Knowledge The Daguerreotype Paper Photography and the Licht-paus, or New Talbot Process The Development of Modern Photography The Negative Process The Positive Process Tjight as a Chemically-Operative Agent Chemical Effect of different Sources of Light On the Eefraction of Light The Photographic Optical Apparatus The Chemical Effects of Light (a) Operation of Light on the Elements (b) Chemical Effect of Light on Salts of Silver On the Correctness of Photographs ... (a) Inflnenoe of the Individuality of the Photographer (i) Influence of the Object, of the Apparatus, and of the Process Light, Shade, and Perspective The Applications of Photography § I. Portrait Photography § II. Landscape Photography § in. Photogrammetry, or Levelling by Photo. graphy § IV. Astronomical Photography II. III. IT. V. VL TII. Tin. IX. X. XI. XII. XIII. XIT. PAGE 1 14 23 31 37 46 55 GS 83 89 105 107 109 120 120 122, 134 149 149 159 166 171 VIU TABLE OF CONTENTS. CHAPTEE PAQB XIT. The Applications of Photography,— co»iii7iue3. § V. The Photographic Observation of Scientific Instnunents ..• ••• ••• "^'^ § VI. Photography with reference to Medical Re- search ... ... ... ..• 202 § VII. Photography and the Microscope ... ... 205 § YIII. MicroBcopio Photographs and the Photo- graphic Pigeon Post ... ... ... 209 § IX. Pyro-photography ... ... ... 212 § X. Magic Photography ... ... ... 214 § XI. Scamoni's Heliographic Process ... ... 215 § XII. Photography and Jurisprudence ... ... 217 § XIII. Photography, Industry, and Art ... ... 219 XV. Chromo-photography ... ... ... ... 221 § I. Chromic Combinations ... ... ... 222 § II. Heliography with Salts of Chromium ... 225 § III. The Production of Photo-reliefs ... ... 230 § IV. Printing in Belief ... ... ... 235 § V. Pigment Printing, or the Production of Charcoal Pictures ... ... ... 239 § VI. Light Printing ... ... ... ... 243 § VII. Aniline Printing ... ... ... 246 *§ VIII. Photo-lithography ... ... ... 248 § IX. Pyro-photography with Salts of Chromium... 257 § X. Photography and the Sand-blowing Process 260 § XI. The Photometer for Chromo-Photography ... 262 § XII. The Chemical Effect of Light and the Pea- Sausage ... ... ... ... 264 XVI. Iron, Uranium, and Copper Photography ... ... 265 XVII. The Change of Glass under the influence of Light ... 269 XVin. Photography in Natural Colours ... ... ... 272 XIX. Photography as a Subject to be Taught in Art and Industrial Schools ... ... ... ... 277 Index ... ... ... ... ... ... 285 LIST OF ILLUSTRATIONS. FIG. PAGE 1. Ivy leaf 6 2. „ copied on stone, prodnoed by nitrate of silver .. 6 3,4. The camera obsonra ... ... 7,8 5. Leaf prints ... ... ... ... ... ... ... 24 6,7. Copying frames for Talbot types 26 8. Flat dish containing water ... 27 9. Zicht.pctus lea£ -piints 29 10. Negative process for transporting the plate ... 43 11. Copper frame to press sensitized paper 50 12. Masks of metal or cardboard ... 53 13. Illufltration of nndnlation ... 57 14. Bays in a drop of water ... ... ... 61 15,16. Spectmm illustrations ... ... C2 17. Tjines in the spectrum ... 63 18. Solomon's lamp 69 19. Dnunmond's lime light ... 70 20. Electric light 71 21. Electric light cylinder ... 72 22. Electric battery 73 23. Electric light apparatus ... 73 24. Reflection in nndergrouud places ... 74 25. Illustration of coloured rays ... 76 26. The earth 78 X LIST OP ILLUSTEATIONS. PIG. PAGE 27. Eefraotion 83 28. Angle of incidence ... 84 29. Angles of incidence 84 30. Triangular prism 85 31. Deviations of rays .,, ... ,„ ... ... ... 85 32. Parallel rays 86 33. Burning glass 86 34. Focus 87 35. Focal distance 87 36. Camera obsoura 89 37. Camera 90 38. Focussing ... ,,. 91 39. Telescopic lens 92 40. Magic lantern ... 93 41,42. Solar camera ,,, 95,96 43,44. Stereoscope ... 99 45. Burning glass ... ... 100 46. Lens 100 47. Prism refraction , 101 48. Stereoscopic deflection , 101 49. American stereoscope 103 50. Lenses used in photography 128 51. Distortore of spheres 124 52. Cube and cylinder 134 53. Bectangles ... ... .. 135 54. Pencils of light 137 55. Intersecting angles ... 138 56. Abnormal appearances produced by divergent lenses 139 57. Portrait taking 140 58 — 61. Foreshortening and distance 143,144 62. Distance in portraits ... 145 63—66. Point of view of the spectator 146, 147 67. Magnifying apparatus 156 6S. Apparatus of landscape photographer 162 LIST OF ILLUSTKAXIONS. XI ria. 69. Revolving camera 70. Trigonometrical surveying 71. Measurement of altitudes by photography 72. Telescope adapted to photography 73. Telescope parallaotically set up 74 — 76. Eclipse of the sun at Aden 77. Photographic view of the corona 78. Appendage to telescope (Rutherford's) 79. Spectral apparatus 80. Distance of heavenly bodies 81. Determining of angles of planets by photography 82. Thermometer and barometer reading by photographs 83. Photographic instrument for ear diagnosis ... 81'. Hehopictor 85. Photographic microscope 86. Magnifying powers ... 87. Ordinary microscope 88. Photographic camera adapted to a microscope 89. Chromo-photography 90. Galvano-plastic apparatus 91. Pantograph 92. Gelatine reliefs 93. Insoluble £lms Si. Photometer PAGE 165 ... 168 170 ... 172 173 182.184 185 187 195 196 198 200 203 204 206 207 207 208 223 228 231 232 234 262 LIST OF PLATES. PLATE I. Photograph of the moon , „. Frontispiece. II. Negatiye and poBitive of the Ucht-jowus process To face page 96 III. Electrotype from heUograph „ 230 IV. Copper-plate from heliograph „ 230 V. Eeduotion of a section of the ordnance Bnrvey map of Wales ... „ 254 TI. Effect of retouching of negatives „ £'15 THE CHEMISTEY OF LIGHT. CHAPTEE I. DEVELOPMENT OP OUE PHOTO- CHEMICAL KNOWLEDGE. Diverse Kinds of Operations of Light — Physical and Chemical Changes — Bleaching Effect of Light — Effects of Light npon Chloride of Silver and Lunar Caustic (Nitrate of Silver) — Chemical Ink — Pictures npon Paper saturated with Nitrate of Silver — ^The Labours of Wedgwood and Davy — The Camera Obscnra — Nifepoe — Effects of Light npon Asphaltum — Heliography — ^It# Application to the Production of Paper Money — Iodide of Silver — Discovery of the Daguerreotype. The light whicli radiates from the great central body of our planetary system produces manifold effects upon the animate and inanimate world, some of which are at once evident to the senses, and have been known for thousands of years, while others, again, are not so ap- parent to the eye, and have been discovered, examined, and utilized only through the observations of modern times. 2 THE CHEMISTEY OF LIGHT. The first effect which every person, however unculti- vated, notices, when, after the darkness of night, the sun rises, is the visibiHty of bodies. The rays from the source of light are thrown back (reflected) from these different bodies, they reach our eyes, and produce an impression upon the retina, the result of which is the perception of material objects through the eye. But soon another effect is observed, not through the eye, but by sensation. The sun's rays not only illuniine bodies upon which they fall, but heat them, as is felt when the hand is held in the rays. Both effects, the shining or illuininating, and the warming effects of the sunbeams, differ very essentially from each other. The iUuminat- ing effect we perceive instantaneously ; the heating effect is only felt after a certain time, which may be shorter or longer, as the heating power of the sun is stronger or weaker. In addition to these two effects of sunlight, there is a third, which generally requires a still longer period to make itself noticed, and which cannot be directly per- ceived either through the eye or by sensation, but only through the peculiar changes, which light produces in the material world. These are the chemical effects ,of light. If we take a smaU piece of wood, and bend or saw it, we change its form ; if we rub it, it becomes warm ; we change its temperature, but it stiU remains wood. These changes, which do not affect the substance or matter {stoff) of a body, we term physical. But, if we set fire to a piece of wood, strong-smelling gases ascend, ashes are deposited, and a black residuum remains, which is totally different from the wood. By this process a quite different substance — charcoal — has DEVELOPMENT OF OUR PHOTO -CHEMICAL KNOWLEDGE. 3 been produced. Material changes of this kind we term chemical changes; — and such chemical changes are, in an especial manner, the result of heat. If, for instance, we heat a bright iron wire red hot, it undergoes ap- parently only a physical (not a material change). But, if we allow it to cool, we find the bright rod has become dull and black ; that it has received a brittle, black surface, which, in the process of bending, easily breaks, and the bright, tough, flexible iron has undergone a chemical change. That is to say, a change of substance has taken place, the iron has been converted into another body, into black oxide, because it has- combined with a component part of the surrounding air with oxygen. Chemical changes of this kind are not only produced by heat, but also by Hght. It has long been known that when the colours of which fabrics are dyed are what is caUed "not fast colours," they fade in the Hght, that is, become paler. In this case the coloured material changes into a colourless or differently coloured body ; and that this is the effect of light is evident from the fact that that part of the material in question which is covered up from the light — ^that beneath the folds — ^remains unchanged. This discolouring effect of light has been long turned to prac- tical use in the bleaching of linen. The unbleached fabric is spread out in the sunlight, and repeatedly moistened vdth water ; and thus, through the combined effect of light and moisture, this dark colouring sub- stance becomes gradually soluble, and can then be removed from the linen by boiling it in lie (alkaU). It was formerly believed that the changes we have just described were caused by the heat which is pro- 4 THE CHEMISTEY OF LIGHT. duced in bodies by the sun's rays. That this is an erroneous view is evident from the fact that fabrics dyed in colours which are "not fast" can be exposed for months together in the temperature of a hot oven without any bleaching effect; and further, that wax, which the sunlight likewise bleaches, becomes darker, rather than paler, through heat. As we remarked before, the bleaching effect of sun- light is a slow process, and this circumstance renders the phenomenon less striking. A sudden and rapid occurrence surprises us, and stirs us up to inquire and to reflect. In the mines of Friburg is now and then found a vitreous duU-shining silver ore, which, on account of its appearance, is called horn silver (muriate or chloride of silver). This horn silver consists of silver and chlorine in chem- ical combination, and can be artificially produced by directing chloric gas upon metallic silver. This horn ore in its original position is completely colourless, but as soon as it is exposed to the daylight it assumes, in a few minutes, a violet tint. This effect of Hght has long excited the astonishment of men of science. In another argentiferous substance this phenomenon is still more obvious. If nitric acid is poured upon silver, it dissolves with effervescence. If the liquid part of the solution be evaporated, a solid mass of crystals is obtained, which is not silver but a combination of silver with nitric acid. This nitric-acid silver is totally dif- ferent from ordinary silver ; it dissolves easily in water, like sugar ; it has a bitter, disagreeable taste ; it readily diminishes and destroys organic matter ; and it is there- DEVELOPMENT OF CUE PHOTO-CHEMICAL KNOWLEDGE. 5 fore used as a corrosive agent, tinder the name of lunar caustic, or nitrate of silver. It has been long known that the fingers which grasp the lunar caustic, or anything that is sprinkled with a solution of it, quickly assume a dark colour. This can he at once tested by moistening a smaU piece of paper with the silver solution, allowing it to dry, and then placing it in the light. These properties were soon made use of to produce a so-called indelible ink, which is nothing more than a solution of one part nitrate of silver in four parts of water, combined with a somewhat thick solution of gum. Written characters traced with it upon linen cloth are pale; but, when dried in the sunlight, quickly become dark brown, and are not injured by washing. Ink of this kind is much used in hospitals for marking linen. A quill, not a steel pen, must be made use of, as that metal decomposes the nitrate of silver. It is not unusual to print the characters by means of wooden type. From the discovery of the blackening of paper saturated with lunar caustic to the invention of photography there was but a step ; yet it was long before any one thought of producing pictures by the help of light alone, and still longer before these attempts were crowned with success. Wedgwood, the son of the celebrated manufacturer of porcelain who produced the still popular Wedgwood ware, and Davy, the celebrated chemist, made the first attempts in the year 1802. They placed flat bodies, such as the leaves of plants, upon limar caustic paper. Light was thus kept from the superimposed parts of the paper, the imderlying parts remained white, whilst the un- 6 THE CHEMISTRY OF LIGHT. covered portions of the paper were blackened by the light; and thus was produced a white outline, or " silhouette," of the superimposed objects upon a black ground. (See Figs. 1 and 2.) Fig. 2. "Wedgwood produced in this manner copies of draw- ings in white lines upon a black ground made upon glass, and this process became the basis, in modern times, of a mode of treatment which attained the highest importance, coming under the name of the lighi-paus process. Unfortunately these pictures were not durable. They had to be kept in the dark, and could only be exhibited in a subdued light. If they remained long exposed to the light, the white parts also became black ; and thus the picture disappeared. No means were then known to make the pictures durable, that is to say, to make them light-resisting, or as we now say, to fix them. But the first step towards the discovery of photography was made; and the idea of producing pictures of the world DEVELOPMENT OP OTJE PHOTO-CHEMICAL KNOWLEDGE. 7 of matter without the help of the draughtsman became, after these first attempts, so extremely attractive that, from that time, both iu England and in Prance, a large number of persons occupied themselves with the subject in private with the greatest enthusiasm. It is clear that by the process of Wedgwood and Davy only flat bodies could be copied, and, notwithstanding all the improvements of which the process was stiU susceptible, it admitted of only a limited application. But even Wedgwood seized the thought whether it were not possible by the help of light to produce pictures of any bodies whatsoever on sensitized paper. He tried to effect this by the aid of an interesting optical instru- ment which had the property of projecting flat-shaded images of solid objects. This instrument is the camera obscura. Kg.s. If a small hole be made in the window shutter of a completely darkened room on a sunny day, a clear image of the landscape will be seen on the opposite wall of the room. Let a be a poplar, o the hole, and w the back wall of 8 THE CHEMISTET OP LIGHT. the room, then from each point of the poplar rays of light are projected towards the hole, and beyond that in a straight line to the wall. It is now clear that to the point a! light can only arrive from the point a of the poplar, which is situated on the extension of the line a ' and 0. Therefore the point in question of the waU can only reflect Hght, which in its colour and position, corres- ponds to the point a. The same remark applies to the points / and g, and the result accordingly is that on the wall an inverted image of the tree is visible. This was first observed by Porta the celebrated Italian natural philosopher ot the 16th century, whose house, we are told by contemporaries, was seldom free from visitors in search of knowledge. This instrument was soon im- proved by substituting a small box (Fig. 4) instead of the chamber, which box in place of a sohd waU had a movable unpolished shde. On this slide the image of an object in front of the box is clearly visible, if a minute hole is made ia the front partition, which answers best if composed of a thin tin metal plate.* These images appear still more beautiful if, instead of * To prevent the access of light the head must be covered with a oloth. DEVELOPMENT OF OUR PHOTO-CHEMICAL KNOWLEDGE. 9 a hole, a glass lens, or, as it is called, a focal lens, is substituted. This focal lens, or " burning-glass," at a certain distance, which is equal to that of its focus, projects a distinct image of the objects — which is much better defined and clearer than that which is produced by the hole. In this improved form was the instrument now em- ployed by Wedgwood and Davy. Their idea was to catch a small picture upon the unpohshed sKde by means of sensitized paper. They fastened a piece of paper saturated with salts of silver upon the place of the image, and left it there for several hours — ^unfortunately without result. The pictures were not distinct enough to make a visible impression upon the sensitized paper ; or the paper was not sufficiently sensitive. It now became necessary to find a more sensitized preparation to catch and to retain the indistinct image; and this was achieved by a Frenchman — Nicephore Niepce. He had recourse to a very peculiar substance, the sensitiveness of which to light was before unknown to any one — asphaltum, or the bitumen of Judsea. This black inineral pitch, which is found near the Black Sea, the Dead Sea, the Caspian, and many other places, is soluble in ethereal oils — such as oil of turpentine, oU of lavender, besides petroleum, ether, and others. If a solution of this substance is poured upon a metal plate, and allowed to cover the surface, a thin fluid coating adheres to it, which soon dries and leaves behind a light brown film of asphaltum. This film of asphaltum does not receive a darker hue in the light, but it loses by light its property of solubility in. ethereal oils. If such a plate, therefore, is put in the place of the 10 THE CHEMISTEY OP LIGHT. small image of the camera obscura, the asphaltum coat- ing will remain soluble on all the dark places (shadows) of the image, whilst on the light spots it will remain insoluble. The eye, it is true, does not perceive these changes. The plate appears the same after as before being exposed to the influence of hght. But, if oil of lavender is poured over the coating of asphaltum it dissolves all the spots that had remained unchanged, and leaves behind all those that had been changed by light, that is, had been rendered insoluble. Thus, after several hours ex^Dosure in the camera obscura, and subse- quent treatment with ethereal oils, Niepce succeeded, in fact, in obtaining a picture. This picture was very im- perfect it is true, but still interesting as a first attempt to fix the images of the camera, and still more interest- ing as evidence that there are bodies which lose their solubility in the sunlight. These facts were again verified long after the death of Niepce, and they led to one of the finest applications of photography, that of teUography, or the combination of photography with copper-plate printing, which Niepce himself to all ap- pearance had already known. A copper-plate print is produced in this way: — ^A smooth copper plate is engraved with the burin (or graving tool), that is to say, the lines which, should appear black in the picture are deeply incised in the plate. In producing impressions, ink is first rubbed into these incisions, and then a sheet of paper is placed upon the plate and subjected to the action of a cyhndrical press ; after which the ink passes over the paper and produces the copper-plate impression. Niepce en- deavojired to substitute, by the help of light, a less DEVELOPMENT OF OUR PHOTO-CHEMICAL KNOWLEDGE. 11 laborious process than the troublesome one of cutting by the engraver. To effect this he covered the copper plate with asphaltum as before stated, and exposed this to the light beneath a drawing on paper. In this case the black lines of the drawing kept back the light ; and, accordingly, in these places the asphaltum coating re- mained soluble ; under the white paper, on the contrary, it became insoluble. Therefore, when lavender oil was afterwards poured over the plate, the parts of the asphaltum which had become insoluble adhered to the plate, whilst the soluble parts were dissolved and re- moved ; and thus the plate in those places was laid bare. Thus a film of asphaltum is obtained on the plate in which the original drawing appeared as if engraved. If a corrosive acid is now poured on such a plate, it can only act on the . metal in those places where it is not protected by the asphaltum; and in these places the metal plate was in fact eaten into. Thus an incised drawing upon a metal was effected through the corrosive operations of the acid, and a plate was obtained which, when rubbed clean, could be used for impressions like an engra,ved copper plate. Copper- plate impressions of this kind have been found amongst the papers left behind by Niepce, which he called "heliographs," and showed to his friends as far back as ,1826. This method, in an improved form, is still in use at the present day, especially in the printing of paper money, when it is requisite to produce a number of engraved plates which are all to be absolutely alike, so that one piece of paper money may perfectly correspond to another, and may therefore be distinguished from counterfeits. In this way the arms 12 THE CHEMISTKY OP LIGHT. and the inscription on the upper side of the Prussian ten thaler notes are printed off from heliographic plates. Thousands of people carry photographic impressions in their pocket-books, without knowing it. Nor is there any occasion to fear that these notes can be imitated easily by the help of photography or heliography. We shall show later on, that the ground tint, the paper itself, and the colour of the inscription present well- devised obstacles to all such imitations, and make them very difficult, if not impossible. Niepce's impressions were undoubtedly very imperfect, and therefore remained unnoticed. He himself gave them up, and again entered upon a series of experiments to fix the charming images of the camera obscura. In 1829 Daguerre joined him ; and both carried on ex- periments in common nntU 1833, when Niepce died without having obtained the reward of his long-continued efforts. Daguerre went on with his experiments ; and he would not, perhaps, have carried them any further if a fortunate accident had not worked in his favour. He made experiments with iodide of silver plates. Then he produced, by exposing silver plates to the vapour of iodine, a pecuHar and very volatile chemical element. Under this treatment, the silver plate assumed a pale yellow colour, which is peculiar to the com- bination of iodine and silver. These plates of iodide of sUver are sensitive to light, they take a brown colour when exposed to it, and an image is soon produced upon them when they are exposed to the operations of light in the camera. A very long exposure to light, however, is necessary to this end; and the thought could scarcely DEVELOPMENT OS" OUR PHOTO-CHEMICAL KNOWLEDGE. 13 have- arisen of taking the likeness of any person ia this manner, for he would have been obliged to remain motionless for hours to obtain it. One day Daguerre placed aside as useless, in a closet in which were some chemical substances, several plates that had been exposed too short a time to the light, and therefore as yet showed no image. After some time he looked by accident at the plates, and was not a little astonished to see an image upon them. He immediately diviaed that this niust have arisen through the operation on the plates of some chemical substance which was lying in the closet. He, therefore, proceeded to take onQ chemical out of the closet after the other, and placed ia it plates recently exposed to the light, when, after remaining there some hours, images were again produced upon them. At length he had removed in succession all the chemical substances from the closet ; and still images were produced upon the plates that had been exposed to the light. He was now on the point of believing the closet to be bewitched, when he dis- covered on the floor a shell containing quicksilver, which he had hitherto overlooked. He conceived the notion that the vapour, from this substance — ^for mer- cury gives off vapour even at an ordinary temperature — must have been the magic power which produced the image. To test the accuracy of this supposition, he again took a plate that had been exposed to light for a short time in the camera-obscura, and on which no image was yet visible. He exposed this plate to the vapour of quicksilver, and, to his intense dehght, an image appeared, and the world was again enriched by one of its most beautiful discoveries. 14 THE CHEMISTEY OF LIGHT, CHAPTEE n. THE DAGUEREEOTTPB. Its Publication and Spread — Ita Path of Development — Improvements — Discovery of the Portrait Lens— Esthetic effects of the Daguerreo- type. Many persons at the present day who have before their eyes the grand productions of paper photography, such, ■ for example, as portraits of hfe-size, view doubtless with pity, or even contempt, the little pictures that were called daguerreotypes from their inventor. The appearance of these pictures was no doubt injured by the ugly mirror- Hke dazzle which prevented a clear view of them. It was different in the year 1830, when Daguerre's discovery was first spread abroad by report. Pictures were said to be produced without a draughtsman by the operations of the sun's rays alone. That was of itseK wonderful ; but it was stiU more wonderful that, by the mysterious operation of light, every body impressed its own image on the plate. How many extravagant hopes and how many evil prognostications were associated with the report of this mysterious invention. It was prophesied that painting would come to an end, and that artists would die of starvation. Every one THE DAGUEREEOTYPB. 15 hoped that he could "with ease obtain images of any objects -which he desired. A friend is leaving home : in an instant his image is permanently retained at the moment of departure. A joyous company is assembled : a picture is taken of it at once as a souvenir. All objects were thus retained as pictures by the chemical action of the rays of light : the landscape glowing with the magical effects of sunset, the favourite spot in the garden, the daily motley move- ment of the streets, of men, of animals, of everything ahve. Then came sceptics who declared the whole thing impossible. These persons were reduced to silence by the testimony of Humboldt, Biot, and Arago, the three celebrated natural philosophers to whom Daguerre dis- closed his secret in 1838. The excitement grew. Through the influence of Arago an application was made to secure to Daguerre a yearly pension of 6000 francs, provided he niade pubHe his discovery. The French Chamber of Deputies agreed ; and, after a long and tiresome delay, the discovery was at length disclosed to the expectant world. It was at a memorable public seance of the French Academy of Sciences in the Palais Mazarin, on tha 19th of August, 1830, that Daguerre, in the presence of all the great authorities in art, science, and diplomacy, who were then in Paris, illustrated his processes by experiment. Arago declared that "France had adopted this dis- covery, and was proud to hand it as a present to the whole world ; " and henceforth, unhindered by the auackery of mystery, and unlimited bv the rieht of 16 THE CHEMISTEY OF LIGHT. patent,* the discovery of Daguerre made the round of the civilized world. Daguerre quickly gathered round him a number of disciples from aU quarters of the globe ; and they trans- planted the process to their homes, and became in their turn centres of activity, which daily added to the number of disciples of the art. Sachse, a dealer in art still living in Berlin, was initiated into Daguerre's discovery on the 2nd of April, 1839, and was appointed Daguerre's agent in Germany on the 22nd of September, four weeks after the publica- tion of the discovery. Sachse had already produced the first picture at Berlin. These pictures were gazed at as wonders, and each copy was paid for at the rate of from ^1 to £2 ; while original impressions of Daguerre fetched as much as £4 16s. 8d. (120 francs). On the 30th of September Sachse made experiments in the Park of Charlottenburg, in the presence of , King Frederick William the Fourth. In October the earliest Daguerre apparatuses were sold in Berlin. The first set of apparatus was purchased by Beuth for the Eoyal Academy of Industry at Berlin ; and it is still to be seen there. After the introduction of the apparatus, it was in the power of every one to carry out the system ; and a great number of daguerreotypists started into exist- ence. Men of science, too, cultivated (more than they do now) the new art : among others, the natural philosophers, Professors Karsten, Moser, Norrenberg, Von Bttinghausen — nay, even ladies, as Frau Professor Mitscherlich. The first objects photographed by Sachse * It was only in England that the process was patented, before its publication on the 15th of July, 1839. THE DAGUBEEEOTYPB. 17 •were architectural views, statuary, and paintings, -which for two years found a ready sale as curiosities. It was in 1840 that he first represented groups of living persons, and in this way photography became especially an art of portraiture. It made the taking of portraits its prin- ciple means of support, and in two years there were daguerreotypists in all the capitals of Europe. In America a painter. Professor Morse, afterwards tho inventor of the Morse telegraph, was the first to prepare daguerreotypes; and his coadjutor was Professor Draper. Let us now consider more closely the process em- ployed in producing daguerreotype plates. A silver plate, as I have said, or in the place of it a silver-plated copper plate, serves as a plane surface for the image. This is rubbed smooth by means of tripoli and olive oil ; and then it receives its highest polish with rouge and water and cotton. It is only a plate so extremely well polished that can be used for the process. This burnished plate is placed with its polished side upon an open square box, the floor of which is strewn with a thin layer of iodine. This iodine evaporates, its vapours come into contact with the silver, and instantly combine with it. By this means the plate first assumes a yeUow straw-colour, next red, then violet, and lastly blue. The plate is then protected from the light ; next it is placed in the camera obscura, where the image on the ground- glass shde is visible, and " exposed" for a certain time. It is afterwards brought back into the dark, and put into a second box, upon the metal floor of which there is quicksilver. This quicksilver is slightly warmed by means of a spirit lamp. At first not a trace of the 18 THE CHEMISTEY OF LIGHT. image is visible on the plate. This first comes out when the vapour of the quicksilver precipitates itself upon the places affected by the light, and the result is in pro- portion to the effectiveness of the operation of light. During this process the quicksilver condenses itself into very minute white globules, which can be very well discerned under the microscope. This operation is called the development of the picture. After the development the iodide of silver, being sensitive to light, must be removed to render the image durable, that is, " to fix it." This is effected by. using a solution of hypo-sulphite of soda, which dissolves the iodide of silver. Nothing more is required after this than to wash with water and dry, and the daguerreotype is completed. Sometimes, in order to protect the picture, it was usual to gUd it. A solution of chloride of gold was poured over, and then it was warmed ; a thin film of gold was deposited, which contributed essentially to the durability of the pictures. A picture of this nature, however, remains always exposed to injury, and requires the protection of frame and glass. Daguerre's first pictures needed an exposure of 20 minutes to the light — too long for taking portraits. But soon after it was foimd that bromine, a rare substance having many poiuts of resemblance with iodine, employed in combination with the latter, produces much more sensitive plates, which required far less time, perhaps not more than from one to two miuutes, for exposure. Many of us, perhaps, still remember the early period of photography, when persons were obliged to sit in the full sunlight, and allow the dazzling rays to fall directly upon the face — a torture which is clearly marked and THE DAGUEEEEOTYPE. 19 visible on the portraits still preserved of tliese photo- graphic victims, in the blackened shadows, the distorted muscles, and the half-closed eyes. These caricatures could certainly not bear any comparison with a good portrait from the hfe, nor probably would portrait-photo- graphy have ever had such success if it had not suc- ceeded in obtaining the exposure to a moderated light. This was obtained by the invention of a new lens — the double objective portrait lens of Professor Petzral, of Vienna. This new lens was distinguished by the fact that it produced a much clearer picture than the old lens of Daguerre, because it was now possible to take impres- pions from less dazzlingly lighted objects. This lens was invented by Petzral in 1841. Voigtlander ground the lens accordiag to his directions, and soon one of Voigt- lander's lenses became indispensable to every daguerreo- typist. By employing iodide of bromium and Voigtlander's lens, the process of exposure was made a matter of seconds. The daguerreotype art had thus reached its zenith. However delicate pictures produced appeared, it was found, after the first enthusiasm had gone, and had given place to a cold spirit of criticism, that much still remained to be desired. First, the gloss and brilliancy of the pictiires make it difficult to look at them. Then there are several marked deviations from nature: yeUow objects often produced little or no effect, or gave a black impression; on the other hand, blue objects, which appear dark to the eye, frequently, though not always, came out white. This is still the case in photography, only now the 20 THE CHEMISTRY OF LIGHT. attempt is made to diminish this defect by subsequent treatment of the plate (negative re-touching). But still a well-grounded aesthetic objection was brought against these pictures. It was indisputable that the daguerreotype greatly surpassed painting by the wonderful clearness of detail, by the fabulous truthfulness with which it reproduced the outlines of objects. The daguerreotype plate gives more than the artist, but for that very reason it gives too much. It reproduces the subordinate objects as faithfully a^ the principal object in the picture. Let us take the simplest case — a portrait. A painter, when he paints a portrait, does not by any means paint all that he sees in nature. The original wears, perhaps, a shabby coat, which shows a good many creases, per- haps a spot of grease, or a patch ; but this does not distress the painter in the least, for he leaves 'out these accidental details. In the same spirit, if the original is seated before a whitewashed wall, the artist by no means puts a whitewashed wall into his picture, for he can leave out all that is displeasing, or add, on the contrary, what he wishes. It is different in photography. This art, in taking portraits, reproduces all those minor accessories which disturb the picture, as faithfully as the principal object in it — ^the individual himself. Another point must be added to this. The different elements admitted by the painter into his picture are by no means made equally pro- minent. The head is the chief consideration in every portrait. The painter accordingly gives his best skill and care to the painting of the head in the most careful manner. At the very least he puts the head in the THE DAGUEREEOTYPE. 21 etrongest light, and leaves the rest of the picture in a half-shade. But in photography it is by no naeans the head which is generally the most prominent — ^frequently it is a chair, or part of a background ; and this detracts considerably from the effect of the picture. Finally, the expression of the face is an important point in a picture; and this varies with the mood of the sitter. Photo- graphy gives the expression vrhich the original had at the moment the picture was taken. Now the expression varies, and is affected by a slight annoyance, a vexatious circumstance, ennui, or even by the motionless attitude which has to be observed during the process ; and hence the portrait often looks strange and unnatural. It is quite otherwise with painting. The painter has longer sittings of the original than the photographer; he soon learns to distinguish the accidental frame of mind from the characteristic expression of the face, and thus he is in a condition to produce a portrait much more closely corresponding with the character of the ori- ginal than that of the photographer can ever be. This naturally applies only to paintings of masters of the first order. In the portraits of the dauber, none of these advantages are found ; and this large class disappeared, like bats before the light, when the art of sun-painting suddenly rose upon the world. Many of these themselves adopted the new art and attained to greater results than they could have done as painters.. The artist of merit has no cause to fear photography. On .the contrary, it proves advantageous to him by the fabulous fidelity of its drawing — through it he learns to reproduce the outline of things correctly — nor can it be disputed that, since the invention of photography, a 22 THE CHEMISTRY OF LIGHT. decidedly greater study of nature and a greater truth- fulness are Tisible in the works of our ablest painters. We shall see further on, how even photography appro- priated the {esthetic principles according to which painters proceed in preparing their portraits, and how thereby a certain artistic stamp was given to these pro- ductions, which raised them far above those of the early period. But this result was only possible when the technical part of photography had been brought to per- fection, and a material better adapted to artistic work than an unyielding sUver plate had been introduced. CHAPTER III. PAPEE PHOTO GEAPHY AND THE LICRT-FATJ3, OE NEW TALBOT PEOCESS. Talbot's Paper Photographs — LicM.Paus Paper— Leaf-prints — Licht- Faus Process and its Application. In the same year that I>aguerre published his process for the production of images on silver plates, Fox Talbot gave to the world a process for preparing drawings on paper by the help of light. Talbot was an English gentleman of fortune, who, like many Englishmen of leisure and means, employed his time in scientific obser- vations. He plunged paper into a solution of kitchen salt, dried it, and then put it into a solution of silver. In this wa,y he obtained a paper which was much more sensitive to light than that employed by Wedgwood. He employed this paper in copying the leaves of plants. Talbot himself says, "Nothing gives more beautiful copies of leaves, flowers, etc., than this paper, especially under the summer sun ; the light works through the leaves, and copies even the minutest veins." This is no exaggeration of Talbot. In the hands of the author there are impressions of this kind, from Talbot's own hand, which show excellently well the venous structure. The pictures copied in this way in the sunlight are 24 THE CHEMISTEY OF LIGHT. naturally not durable, because the paper, by having salts of silver in its composition, is still sensitive to light. But Talbot offered the means of fixing the pictures — he plunged them in a hot solution of kitchen salt ; in this way the greater part of the salts of silver was removed, and the pictures did not become obscure to any considerable extent in the hght. Fig. 5. The celebrated Sir John Herschel carried out this fixing process even more successfully by plunging the pictures into a solution of hypo-sulphite of soda. This salt, which dissolves all the salts of silver, was at that time very expensive, costing 6 shillings per pound. The production of this salt soon kept pace with the increasing PAPEE PHOTOGEAPHY AND THE "lIOHX-PAUB" PEOCESS. 25 demands of photography, and now it is offered for sale by the ton, and at as low a rate as 6^^. the pound. By this means the production of a durable sun-picture on paper, which Wedgwood had in vain attempted, was rendered possible. This method gave, no doubt, only pictures of flat objects which could be easily pressed on paper ; for instance, leaves of plants, patterns of stuffs. The process has lately been resumed, after it had almost been forgotten. Charming ornaments of leaves, different plants, and flowers were produced ; and these copies were proportionally more beautiful than the earlier ones, because a much finer and even-surfaced paper than that of Mr. Talbot has recently passed iuto trade, under the name of licht-pdus papier.* These priats are much liked in America. We give on the accompanying page a faithful imitation of one of these leaf-prints. Since, by the cheapness of sensitive paper, the pro- duction of these leaf-prints has been made very easy, we give here the mode of producing them for our fair readers, who ^^ill be able in this manner, like then- bisters in America, to make ornamental pictures for the adornment of lamp shades, portfolios, and similar things. The leaves — especially ferns and the like — are suitably chosen, pressed between blotting-paper and dried, then gummed on one side and grouped gracefully by the fair artist upon a glass slab or plate, in a small frame (Fig. 6): As soon as the whole is dry, the impression can be at once commenced.! • This paper is produced by Mr. Eomain Talbot, 11, Karlstrasse, Berlin. + These wooden frames, called copying frames, dishes, and fixing salt, are also mantif aotnred by Mr. Talbot, at Berlin. There is now even a small plaything of this kind on sale, known by the name of the " sun-oopying machine." 26 THE CHEMISTRY OP LIGHT. A small piece of sensitized paper is placed on the leaves after they are arranged, the two wooden hds, h h, are laid upon it, and fastened down by means of two little cross-bar pieces of wood, x x, and then the whole is exposed to the light, the glass side upper- most. The sheet of paper very soon assumes a brOwn colour, where it is not covered by the leaves, and Kg. 6. ultimately it receives a decided bronze tint. The light also penetrates partially through the leaves, and colours the paper lying under them brown. It is easy to discern how far the colouring has passed under the leaves, if one of the cross-bars, x, and the half cover, h, are removed, and the paper is lifted up. Fig. 7. As soon as the impression is dark enough — it is quite a matter of taste whether the shade be dark or light — the paper is taken out and placed for a time in a dark PAPER PHOTOGRAPHY AND THE " LICHT-PAUS " PROCESa. 27 closet. Several pictures can, in like manner, he taken one after the other, and these can be afterwards fixed, that is, made permanent in the light. To this end the picture is placed in a flat dish (Fig. 8) containing water, for about five minutes, and then in a second dish in which a solution of twenty grammes of fixing natrium have been combined with a hundred grammes of water. The moment the impression is dipped in. this it becomes of a yellowish brown. After the impression has remained ten minutes in the fixing solution — several leaves can be immersed in succession — it is taken out and placed in fresh water (most con- veniently in a saucer). This operation of placing in fresh water is repeated from four to six times, the picture being left in the water three minutes each time. Kg. 8. Afterwards the pictures are placed on blotting paper and suffered to dry ; they can then be pasted upon card- board, thick paper, linen, glass, or wood. To many persons this process will appear an agree- able pastime, but latterly it has gained increasing con- sideration as an aid to the copying of drawings, maps, plans, copper-plate impressions, and so forth. This work of copying, which used to cost the artisan and artist many hours of time and labour, and yet was inaccurate, can be accomplished with the least possible trouble by the help of the process described above. 28 THE CHEMISTRY OF LIGHT. Let the reader imagine a drawing placed on a piece of sensitized paper, and, after being firmly pressed together by a glass slab, exposed to the light. The light penetrates through all the white places of the paper, and colours brown those parts of the paper lying under them ; but the black lines of the drawing keep back the light, and thus the underlying paper becomes white in these places. Therefore, if sufficient time is given for the operation of the light, a white copy on a dark brown ground is ob- tained in this manner, which is fixed and washed exactly like the leaf-prints described above. This copy is reversed with reference to the original, like an object and its reflection in a mirror ; in other respects it is a faithful representation, stroke for stroke. We give in Plate II. the copy of a woodcut struck off according to this method. This copy is rather too small, but the largest as well as the smallest drgdwing can be copied equally weU; and copies of this kind, from drawings of the size of 4'H feet, are made in technical offices, in mines, and in the manufactories of machinery. Large copper frames are used for this purpose, re- sembling in their construction the small frames named above ; and large wooden dishes, covered with a coatiug of asphaltum, are employed for fixing and moistening. This operation is called in- practice licht-paus process. The black copy produced by it is called a negative picture, but a second white copy can be prepared from this by placing the negative upon sensitized paper ; then the light shines through aU the clear Hnes, and colours the paper lying under them of a dark hue, whilst it remains white under the dark places of the negative. PAPER PHOTOGKAPHT AND THE "lIOHT-PAUS" PBOCESS. 29 In this raanner a picture is produced which perfectly resembles the primary original, called a positive. The washing and fixing are carried out just as scrupulously with the positive as with the negative. Fig. 9 offers a positive of this kind taken from the negative, Fig. 6. Thus the geographer is in a position to prepare quickly faithful copies of his sketches and maps, the engineer Fig. 9. is able to copy the drawings of machines which are to serve as models for the workmen, and the student can copy illustrations of natural history which are to assist him in his studies. In the process of copying, the sensitized paper — licht-paus paper — ^must closely touch the original picture ; therefore the former must be placed 30 THE CHBMISTBY OP LIGHT. on the side of the picture, and not on the reverse side of the original. This process has already done good service in military operations, where it was important to make quickly a copy of some map of which there was only one impres- sion. If an attempt had been made to draw a copy of the map, it would have required several days to carry out, nor would the copy have been as correct as the licht-paus. It is remarkable that this process, so important for industry, has only quite recently been known in its full value, although the experiments of Talbot have been before the world for thirty years. The explanation of this fact is, no doubt, to be found in the circumstance that th^ paper impressions were far less distinct than now, being often rendered worthless by spots. Another reason has been that the preparation of the paper requires especial care, and therefore frequently failed in the hands of the inexperienced; that is, of those who were not professional photographers. Further, the papers prepared according to the old method soon spoil, and had on that account to be used immediately after they were prepared. These disadvantages have been removed by the inven- tion of Eomain Talbot's licht-paus paper, which is sold ready prepared, and can be kept for months ; and by this means the process can be easily made available by every professional man and amateur. CHAPTEE IV. THE DEVELOPMENT OF MODERN PHOTOGEAPHT. Talbot's Paper Negatives — Photography as an Art for Multiplying Copies — Services of Nilpoe de St. Victor — White of Egg Negatives — Guri-Cotton in Photography — Collodion — Archer's Negative Pro- cess — White of Egg Paper — Carte de Visite — Photographic Album. The reader has already learnt in the previous chapter what constitutes a negative, and how by its means copies produced by light, of plane objects, can be obtained. Talbot, the inventor of this paper process, carried out further researches, in order to represent on paper, by the help of the camera-obscura, material objects which cannot be pressed together with sensitized paper; for example, a person or a landscape. He attained this object two years after Daguerre's discovery, by means of paper prepared with iodide of silver. He saturated paper in a solution of nitrate of sUver, and then in a solution of iodide of potassium. He thus obtained a slightly sensitized paper, but one that could always be rendered very sensitive, by plunging it into pyrogallic acid and silver (gallate of silver).* * The nature of this peculiar process is explained farther on. 32 THE CHEMISTRY OF LIGHT. When this paper was exposed to the light in the camera ohscura, it did not give at once a picture — this was only clearly defined after lying some time in the dark, or by subsequent treatment with pyrogaUic acid and silver — but it came out as a negative, and not as a positive. Thus in taking, for example, a portrait, the shirt ap- peared black, also , the face ; while the coat, on the contrary, came out white. The picture was made light-resisting by plunging it in a solution of hypo-sulphite of soda. A negative thus obtained is a picture on a plane surface of a solid object, and Talbot prepared positive . pictiu'es from negatives of this kind. He placed the negative upon a piece of sensitive paper saturated with chloride of silver, as described in the last chapter, and let the light work upon it. This shone through the white places of the negative, and imparted a dark colour to those parts of the sensitive paper lying under them, while the dark places of the negative pro- tected the paper lying under them from the effects of the light. Thus he obtained a positive picture from a nega- tive. He was now able to repeat the process as often as he pleased, and therefore was in a position to copy, by means of the light, many positives from a single negative. Photography was thus classed among the arts that repeat copies, and this circumstance exercised an im- portant result on its future development. Daguerre's method only gave a single positive at a time ; if more were required, the person had to sit several times. In Talbot's method a single seance sufficed to produce hundreds of pictures. It must be admitted that the earher pictures of the THE DEVELOPMENT OF MODERN PHOTOGRAPHY. 33 Talbot process were not remarkably engaging. Every roughness of the paper and each small spfeck of dirt vrere imprinted on the positive, which could not be compared in point of delicacy with the fine daguerreo- types ; but the method was soon improved. Niepce de St. Victor, nephew of Nicophore Niepce, the friend of Daguerre, had the happy idea of substi- tuting glass for paper. He covered glass plates with a solution of white of egg, in which iodide of potassium was dissolved. A solution of this kind can be easily produced by beating up the white of egg to the consistency of snow, and allowing it to deposit. The glass plates, after being dried and covered with a coating of white of egg, were afterwards dipped in a solution of silver. Iodide of silver was formed in this manner — the whole coating of the white of egg was coloured yellow, and became very sensitive to light. Niepce put these glass plates into the place of the picture in the camera-obscura, and suffered the light to work upon it. Its impression was at first invisible, but afterwards became clearly perceptible when the picture was im- mersed in a solution of pyrogaUic acid. . Thus Niepce obtained a negative on glass without the blemishes which appeared on paper negatives. He repeated this negative exactly according to the same process that had been employed by Fox Talbot, and he obtained from the fine negative a correspondingly fine positive, which was much better calculated to bear a comparison with the productions of Daguerre. Niepce invented his method in 1847. It excited much 34 THE CHBMISTET OF LIGHT. attention, but had a shady side : the treatment with white of egg, salts of silver, and pyrogallic acid was a dirty process. Therefore the method appeared to many, who had been accustomed to the daguerreotype, dirty and unpleasant, and deterred persons from trying it. On the other hand, the advantages of the new process in repeating impressions was so evident that it could not be overlooked; therefore, even those who had an antipathy to soil their fingers nevertheless zealously devoted themselyes to the work. The easy decomposition of white of egg was, however, a great disadvantage in the new process. They sought to avoid this by adopting a more durable substance. This was afforded by a new discovery, gun-cotton, made by Schonbein and Bottcher in 1847. Schonbein found that ordinary cotton saturated in a mixture of nitric acid and sulphuric acid assumes explosive properties similar to those of gunpowder. It was conceived that this substance was an important substitute for gunpowder, but it was soon found that its explosive property was very imequal, being sometimes too strong and at other times too weak. On the other hand, another very useful property was observed in the same body — ^that of being dissolved in a mixture of alcohol and ether. This solution leaves behind it a transparent membrane form- ing an excellent sticking-plaster for wounds. Thus the same substance that was destined to be .a substitute for gunpowder, as a destructive agent for producing wounds, became actually a remedy for the latter. This solution of gun-cotton was caUed coUodion. The thought occurred to different photographic ex- perimenters to try this substance instead of the white of THE DEVELOPMENT OP MODERN PHOTOGRAPHY. 85 egg, by coating glass plates with it ; but the attempts did not at first lead to any satisfactory results. At length Archer published in England a fuU description of a collodion negative process surpassing in the beauty of results, in simplicity and security, Niepce's white of egg process. Archer coated glass plates with collodion, in which salts of iodide had been dissolved ; he plunged this in a solution of silver, and thus obtained a membrane of coUodion saturated with sens-itive iodide of silver, which he then exposed in the camera. The invisible impression of the light thus produced became visible by pouring gallic acid over the plate, or the still more powerful chemical agent, pyrogallic acid ; or, instead of this, a solution of green vitriol. A very delicate, clear negative was directly obtained by this process, which yielded much more beautiful impressions on paper than the original negative paper of Talbot. A very essential improvement was sub- sequently made in the preparation of negative paper by coating it with white of egg, according to the process of Niepce de St. Victor. By this means it received a brUHant surface, and when exposed to the light it took a more beautiful and warmer tone, which gave the pictures a brighter appearance than those produced upon the ordinary paper. Thus the Talbot-type, which at first seemed hardly worth notice compared with the process of Daguerre, was gradually so perfected by successive improvements, that it ultiaiately took precedence of Daguerre's. After 1853, paper pictures on collodion negatives came more and more into vogue, the demands for daguerreotypes 36 THE CHEMISTRY OF LIGHT. fell off and soon Tanished altogether, and were produced only here and there in America. The collodion process is now the one universally employed. It acquired an immense impetus through the introduction of cartes de visite. These small por- traits, which are intended to be given away, and there- fore had to be produced in large numbers, were invented by Disderi, the court photographer of the Emperor Napoleon, and obtained so great a success that they were immediately introduced into all circles, and soon became a necessity for everybody. The moderate price at which these portraits were sold made them attractive to the smallest purses, and the general public crowded to the ateliers, the number of which increased daily. The old-fashioned album, the favourite souvenir of young people, was now superseded by the carte de visite, and the portraits of friends were substituted for their written words. A photographic album is now found ia every home; and in Berlin alone there are at present more than ten photographic album manufactories, to satisfy the demand, from whence they are exported to all parts of the world. CHAPTEE V. THE NEGATIVE PEOCESS. Tlie Dark Chamber — Light Inoperative Chemically — Plate-cleaning — Application of Collodion— Sensitizing — The Camera— The Arrange- ment — The Exposure to Light — The Developing Process — The For- tifying Process — The Fixing Process — The Varnishing. In the previous chapters we have dwelt on the develop- ment of this art, and we are now able to feel at home in the atelier of a photographer. His whole business depends on the chemical operation of light, — and yet the scene of his priiicipal activity is not the illuminated atelier, but a -dungeon, in which the deepest night pre- vails, and which is called the dark chamber. The sensitized plate, which has to be exposed to the light, and to respond to its most delicate operations, must be generated in darkness, in the dark chamber. This space, surrounded by bottles and boxes, and crammed with instruments, is the narrow world of the photo- grapher, out of which he issues only for a few minutes into the light of his ateUer, in order to return directly with his illuminated plate, and to subject this to various other chemical operations. Many persona believe that the opening and shutting 3 38 THE CHEMISTBY OF LIGHT. of the slide — the cover or lid of the apparatus falsely called the machine — is the chief -work of the photographer. Nay, it is related of a certain queen, that she thinks she is photographing, when she has all the necessary apparatus brought and prepared, and then, when all is ready for the result, opens and shuts the lid of the objective — a work that a child of five could do equally well. But this operation is only a Unk in a great chain of twenty-eight operations, through which each plate must pass to produce even a negative, while at least eight further operations are required to throw off a positive from this negative. Let us look a little closer at these operations. The appearance of a dark chamber is by no means inviting. Even where the greatest order prevails, drops of solution of silver are diffused about, and black spots produced here and there. To this must be added a permanent odour of the vapour of collodion, and an unavoidable dampness from the necessary washing of the plates, — and all this is seen in the hazy chiaro-oscuro of a gas or petroleum lamp provided with a yeUow shade, or of a small window fitted with a similar glass shade. The remark must here be made at the outset, that the dark chamber of the photographer is not really completely dark. The light of day only must be ex- cluded from certain operations ; but the yellow light of the lamp is innocuous. From this we learn the important distinction between light chemically operative, and light chemically inopera- tive. The light of the sun and of the blue heavens, the electric light, and the magnesium, are chemically very operative, gas light and petroleum light very slightly so ; THE NEGATIVE PEOOESS. 39 •whilst the yellow light of a spirit lamp, whose wick has been rubbed with kitchen salt, is entirely inoperative. The operative light of day, furthermore, can be rendered inoperative if allowed to pass through a yellow or, better still, a reddish-yellow glass shade. The light, therefore, that falls through the yellow window of a dark room is chemically inoperative, or in so slight a degree operative that it no longer causes any disturbing effect. It is re- markable that the yellow light which affects our eyes so powerfully, should influence the photographical plate hardly at all. Up to the present time this fact has not been sufficiently explained. It has disadvantages for practical photography; for example, a yellow garment becomes easily black in photography, a yellow complexion, yellow spots — such as freckles — appear almost black in the picture. Nevertheless, these dis- advantages can be obviated by employing the negative retouche described at a future page. On the other hand, the inoperative property of yellow light has also its advantages for the photographer. It permits him to prepare the sensitive plates in a subdued hght which does not injure them, and yet suffer his eyes to control the work. If the plates were sensitive to all kinds of light, it would be necessary to prepare the plates in absolute darkness, which would be very incon- venient. The first operation required in preparing a sensitive plate — an operation which requires great care — is the cleaning of the glass. The slides, after being cut by the diamond, are placed some hours in a corrosive fluid — ^nitric acid — and by this means all impurities adhering to the surface are destroyed. 40 THE CHEMISTRY OF LIGHT. The acid adhering to it is removed by washing, and the plate is then dried with. a clean cloth. To the uninitiated it would then appear perfectly clean, but the photo- grapher subjects it to further polishing, by rubbing with a few drops of spirits of wine ; or, still better, of ammonia. Each touch with the finger or rub of the sleeve of the cleaned surface, each drop of saliva which might chance to escape from the mouth in coughing, would spoil tne poHshed surface ; nay, even the atmospheric air produces with time disadvantageous effects. If a cleaned plate is left only twenty-four hours in the air, it gradually attracts its exhalations, and another cleansing is rendered necessary. The cleaned glass is saturated with collodion. The collodion itself is, as we know,, a solution of gun-cotton in a mixture of alcohol and ether, to which metallic iodine and bromine — for instance, iodide of potassium and bromide of cadmium — ^have been added. This solu- tion must, also be produced with the greatest attention to cleanliness ; and, in order to preserve the purity of the materials employed, the mixture must be allowed to stand a long time, and the sediment carefully cleared of aU fluidity. The coating of a plate with collodion is an affair of dexterity, and only succeeds with those who have witnessed the process and after some practice. It is usual to hold the corner of the plate with the hand and to pour over the centre of it a circular mass of the thick fluid, and then to allow this to flow to all of the four corners by a gentle inclination of the plate in different directions, and thus ultimately to let the fluid flow off at one of the corners. A considerable part of the fluid originally poured THK NEGATIVE PROCESS. 41 upon it — ^that is, nearly lialf — ^remains, and adheres to the plate. In the process of flowing off, streaks are usually formed, -which would Ukewise spoil the picture; and therefore the plate, whilst being drained, must he con- stantly kept in motion until the last drop has run off. The fluidity stiffens into a soft, moist, spongy film. At the moment when this thick film has become stiff, the plate must at once be immersed in the solution of silver (silver bath). And now a somewhat unusual action of the fluids takes place, for the fihn of collodion repels like fat the watery solution of silver, and a steady agitation of the plate in the solution is necessary in order to make the solution adhere to the plate. This mechanical operation is accompanied simulta- neously by a chemical change. The salts of iodine and of bromine that exist in the collodion film change their properties with nitrate of silver, and give birth to iodide and bromide of silver, and to nitric-acid salts. This iodide and bromide of silver colours the film yellow; and it is only now that the plate is prepared which serves as the ground of the picture about to be painted by the light. All of these operations must precede the taTdng the photograph, and they are begun, in fact, at the moment when the person enters the ateUer, and with proper management the plate is prepared before the arrange- ment for taking the portrait is concluded. This arrangement is a labour of itself; and it is of a genuinely artistic nature. The points to which the photographer has to attend include a natural and yet 42 THE CHEMISTRY OF LIGHT. graceful attitude of the original ; the choice of the side which presents the most advantageous aspect; the picturesque arrangement of the dress ; the removal of inappropriate objects -which ought not to appear in the picture ; the addition of those that are suitable, such as a table, a cabinet, or a background; lastly, an appro- priate direction of the light. Only a few minutes can be devoted to these arrangements, for persons cannot endure long delays or experiments ; and the plate itself only lasts a short time in the sensitive condition, for it is damp through the adhering solution of silver, which soon dries up, and the plate is then useless. When the exposure to the Hght has been accom- plished — a process during which the person being photographed must remain perfectly quiet — the sensitive plate is brought back into the dark chamber. For the purpose of transporting the plate, which must evidently be guarded very carefully from the daylight, the photographer employs a little flat box (Fig. 10), called the cassette, whose floor, H, and cover, D, can be drawn out and closed. In the corner there are silver ledges on which the plate lies ; a wedge fastened to the upper lid keeps them in their place. Thus they can be easily carried in the closed cassette and placed within, the camera obscura ; there its ground-glass sUde is moved to and fro until the picture clearly appears upon it. After the exposure to light has taken place the plate is taken back in the cassette to the dark chamber. And now follows one of the most important operations, the development of the picture. Upon the plate there is as yet no trace of a picture visible. The operation of the light consists in quite a peculiar change of the THE NEGATIVE PEOCESS. 43 iodide of silver which forms the principal constituent of the plate. This iodide obtains through the light the property of attracting pulverized silver, if this has been precipitated on the plate in any shape. This precipitate is produced by the following operation. If a solution of silver is mixed with a very diluted solution of green vitriol, there results by slow degrees a precipitate of metallic silver — not, however, as a green shining mass, but as a grey powder. A solution of silver adheres Fig. 10. now to the sensitive plate resulting from the bath. If after this a solution of green vitriol is poured upon it, a silver precipitate is also occasioned, and the picture is seen suddenly to make its appearance, by the silver powder adhering to the part exposed to Hght. The features of a portrait that are first visible are the lightest — the shirt, then the face, and lastly the black coat. The negative thus obtained, however, is by no means completed by the operation. The picture is usually too attenuated to answer in the 44; THE CHEMISTEY OF LIGHT. positive process for the production of a paper impression with the help of light ; for the production of such an impression results from the light shining through the transparent places of the negative, and colouring dark the places underlying them, -while it is repelled fro^ the parts which, have to remain black. The parts in qu-estion of the negative must be sufficiently transparent to produce this effect. The impression must, therefore, be more strongly defined ; and this takes place by repeating the developing process. A mixture of green vitriol and of a solution of silver is poured upon the picture, and a silver precipitate is formed again on it, adhering only to the lines of the picture, and therefore giving these an intenser colouring. If the plate is not perfectly clean in the processes of developing and defining, silver is precipitated upon the dirt stains and produces spots. After the defining of the picture, or the so-called fortifying process, has been com- pleted, it is only necessary to remove the iodide of silver, which diminishes the transparency of the clear parts of the plate. Then a solution of hypo-sulphite of soda is poured on the plate. This salt has the property of dissolving insoluble salts of silver, so that the iodide of silver vanishes under the influence of this solution. This is the fixing process. Lastly, the plate is washed and dried. If it be borne in mind that all these different operations are performed on a little film, liable to be injured by the least contact of any kind, it is not to be wondered at that in treating matters of such a delicate nature the inexperienced beginner has to destroy so many coatings before a proper one is prepared. Even when dried, the picture is very liable to injury ; THE NEGATrra PEOCESS. 45 and therefore photographers, in order to protect it, cover it with a varnish, that is, -with a solution of a resinous nature, such as shell-lac and orpiment, or red arsenic in spirits of mne. The fragile glass negative is thus brought to completion. This sketch of the operations which a photographer is ohhged to carry on in order to produce a negative, is sufficient to show that photography is a more difficult art than some persons imagine, and that it requires something more than the opening and shutting of a lid. The chief requisite for the success of these operations is routine, that is, the unfaiHng accuracy obtained in practising each part of the process. Faults that are made in any particular operation of the process are, as a general rule, irremediable; and therefore it is absolutely essential to avoid them. 46 THE CHEMISTRY Or LIGHT. CHAPTEE VI. THE POSITIVE PEOCESS. Character of the Negatire — Departure from Nature — Negative Befouche — Prodnction of Sensitive Paper — Striking off Impressions— Toning with Chloride of Gold — Fixing — Canse of Fading — Quantity of Silver in the Picture — Toning down of Photography. In the preceding chapter we have become acquainted "with the production of a negative from nature. However interesting such a negative might be, nevertheless, it could not satisfy the purchaser of a portrait, because it showed everything reversed. The white face it made black, and the black coat, light. No one would hang up on his wall a picture representing him as a Moor. It was therefore necessary to obtain a positive impres- sion from this negative. We have already learnt how this is effected in the chapter on the " Licht-paus " process. It is the old Talbot method that is here em- ployed. But we must still mention some very important collateral operations which are of high significance in modern photography. The camera, the negative process, and the photo- grapher who knows how to manipulate intelligently, no doubt produce a negative which, laid over sensitive paper and exposed to the light, yields a positive ; but THE POSITIVE PROCESS. 47 although this positive is very faithful in the delineation of figures — that is, of their outline — ^it yet presents marked departures from nature. It is especially evident that the relations of light and shade are hy no means correctly given. In general the hght parts appear too light, the dark parts too' dark — as, for example, the folds in a dress, the sHn, and, moreover, the shading under the eyes and chin. When photographers knew nothing of art, these defects were taken as a matter of course. People protested that photography was correct because nature, through photography, was herself the artist. But in this conclusion the co-operation of the photo- grapher was overlooked. No doubt nature^ that is, the object to be taken — makes an impression upon the plate, by the Hght issuing from it ; but an impression of light is not a picture — ■ it is indeed, of itself, invisible ; nay, more, the strength of the impression of light is entirely at the discretion of the photographer, who can make it weak or intense by a greater or less exposure. There is no rule which determines the length of time a photograph has to be exposed to the Hght. The fact is that nature, properly speaking, only determines- the outline of the picture, while the relations of Hght and shade depend partly on those distinctions of natm-e, and partly on the good pleasure of the photographer. The print or impression of the light is developed; hence it becomes visible, and finally the developed picture is brought more strongly out. By this means the photo- grapher can at his option increase, and even exaggerate, the contrasts of light and shade. 48 THE CHEMISTET OF LIGHT. If the negative is carefully compared with the original, we shall find that many dark parts have not appeared at all, because the exposure was too limited for them to produce an impression upon the plate ; others have ap- peared, but too indistinct. On the other hand, very clear parts — for instance, the shirt-collar — have an excess of clearness and whiteness, and the needlework upon it is invisible because the time of exposure was too short. In the case of long exposures it is often remarked that clear parts differing little in colour are entirely con- founded, that is to say, form a single white patch. Moreover, the accessories which a paiater would xm- doubtedly omit, such as warts, pockmarks, little hairs, are all as clearly defined as the principal features ; and thus the negative is neither a correct nor an agreeable repetition of the reahty, but produces in the positive a picture which shows considerable departures from nature, and is often inaccurate by giving too much prominence to accessories. In the first period of photography these departures were overlooked. Every, one was content to possess a portrait which at least showed the outlines correctly; and what was defective in the negative it was sought to atone for through the retouch of the positive. But this retouch rendered the picture dear ; and as it began to be the custom to order pictures by the dozen, the endeavour was made to evade this labour, which had to be apphed to each individual picture, by carrying it out in the negative. A single touched-up negative gave hundreds of un- exceptionable impressions which did not require to be retouched, and thus the negative retouche became the THE POSITIVE PEOOBSS. 49 first and most important operation to produce a faithful and agreeable picture. The essential characteristic of this negative retouche consists in entirely covering several parts. For example, the frecMes and warts in the clear negative are entirely renioved by the pencil, or Indian iak. Other parts — for example, the too delicate details of the hair — are more defined by pencil strokes. Many shadows — for example, the vrrinkles in the face — are softened off by sUght touches of Indian ink. This labour must always be carried out with the thought that all which the painter draws on the negative with his black- lead pencil will appear the opposite ; that is, light in the positive. It is requisite, therefore, for the negative retouche to have a thorough knowledge of working with lead-pencil and Indian iak, to render the different shades in the positive process. The best draughtsman and painter is on that account still far from being able to retouch a negative. It is to be remarked that the negative retouche may, under certain circumstances, go too far. By covering each wrinkle he can make an old face young ; he can beautify an ugly original by cutting away a hump on the back, or other abnormal growths ; and these tricks are often put into requisition for the vanity of sitters, and are dearly paid for. Plate VI. (p. 245) represents two portraits of the same person, one after a retouched negative, the other after a negative that had not been retouched. They repre- sent a celebrated singer (Mdlle. Axtot) ; the spots on the skin and the dark shadows, on the picture which is not retouched, are clearly to be seen, whUe in the re- touched one they are not visible. 50 THE CHEMISTRY OF LIGHT. In many cases the negative retouche is a concession to human vanity, but this is by no means always the case. As already explained above, photography does not always reflect correctly the natural colours. Yellow often becomes black, and blue, white. Therefore, in producing a picture of brighter hues, photography is often very deficient in the reproduction of their tones. Then the negative retouche is a powerful aid to correct this fault, and through this alone have photographs taken from oil paintings attained their present perfection. We will treat of this subject in a future chapter. Let us now consider the operations of the positive processes. The first operation is the production of the sensitized paper. A piece of paper coated with white of egg and moistened with a solu- tion of kitchen salt is laid in a cup with a solution of silver. The paper floats upon the liquid, it sucks it up, and chloride of silver is formed,, through decomposition with the kitchen salt. At the end of a minute the paper is taken out of the silver solution. The wet paper is but slightly sensitized; it becomes fuUy sensitized only after being dried. The dry paper, saturated with chloride and nitrate of sUver, is then pressed together in the copper frame (Fig. 11), which is similar to the one described at a previous page. Then the whole is exposed to the Ught. The same process THE POSITIVE PEOCESS. 61 ensues ■which we have described in the chapter on "lAcht-paus paper " ; the light shines through the clear places of the negative and colours dark the paper lying under them, but the paper under the dark places of the negative remains white^ while it assumes a slight colour under the half-tones. In this maimer a faithful positive reprint of the negative is produced, presenting a beautiful violet-brown tint. We know from the description of the licht-paus process, that this reprint would not stand the Hght long, because the paper is still sensitive to light. The salts of silver contained in it must be removed if the impression is to be made lasting. To this end a solution of hypo-sulphite of soda must be employed. If the impressions be plunged in this solution, they become durable in the light ; but, un- fortunately, by thus dipping them, they suffer a peculiar change of colour, assuming an ugly brown tint. This tint is no injury in technical and scientific pictures, but detracts greatly from portraits and landscapes ; and in order to give these a more agreeable tint before fixing them, they are plunged into a diluted solution of chloride of gold. This process is called toning down. In this operation a part of the gold is precipitated on the outlines, giving to these a bluish shade ; and now, after plunging it into a solution of fixing sodium, the tone of the picture is not essentially altered. The picture thus produced consists partly of gold, partly of silver, in a finely powdered state, and only requires to be thoroughly washed in order to estabhsh its durability. If this washing is omitted, small particles of fixing sodium holding sulphur in suspension remain behind, and these become decomposed and form on the 52 THE CHEMISTEY OP LIGHT. picture yellow sulpHde of silver. This accounts for the fact that the pictures of an earlier period, when from ignorance of these results this thorough washing was neglected, so often turned out pale and yellow. It is surprising what a small amount, of silver and gold is required to give an intense colour to a whole sheet of paper. For in a sheet of this description — 1*447 feet by 1'546 feet — which has become completely blackened, there are only 1"3162 grains ; whilst in a picture of this size there are only 0"075, that is, about one-thirteenth of 15'440 grains,* and in a carte de visite one-five-hundredth of 15'440 grains t of silver. It must be here remarked that pictures which are fresh when printed, pale a little in the fixing process; and hence the photographer usually prints these darker than they ought to remain. Accordingly, the printing process requires a practised eye, simple as it may appear. In certain cases tricks of art are employed to produce agreeable effects, and among these is that of toning down. Our readers are no doubt well acquainted with portraits on a white ground, the outlines of which gradually become confounded with the ground tint of the picture. This effect is produced in a very simple manner by placing what is called a^ mask on the copying frame. This mask is a piece of metal or cardboard (Fig. 12) in which an oval hole h is cut. This is placed on the copper frame K K, so that the part of the negative which shall be impressed on the picture lies perpendicularly under it. This part is then affected * One-thirteenth of a gramme = 1-187 grains. "f- One-five-hnndredth of a gramme = ■031 graina. THE POSITIVB PEOOESS. 53 perpendicularly by the broad bundles of light 8 S, and intensely coloured, while the collateral parts lying under the mask are affected only by the small patch of light, S' S'. Accordingly, they only give a pale reprint on the paper, as they are remote from the margin of the mask. Thus a gently vanishing margin is produced, looMng very artistic, and yet only the result of a very simple trick of art. The picture which the photographer produces in the manner now described only req^uires some rectifying to be an elegant drawing-room ornament. It is cut in a regular shape, square or oval, and fastened with clean paste to white cardboard, and finally, after drying, and the removal of Httle blemishes, by slightly touching up Fig. 12. with the paint brush, it is rendered glossy by two smooth steel rollers, and receives a satin-like surface. Certain sizes have been adopted through custom by the pubhc. Among these is the shape of what is called the "carte de visite," and " cabinet size." The former is rather larger than an ordinary visiting card. The latter is two and a-half times as large. The carte de visite was introduced at Paris by Disderi, in 1858, speedily secured admirers, and has been diffused over the whole earth. Even chemical photo- Si THE CHEMISTRY OP LIGHT. graphers prepare photographs in the form of cartes de visite. The carte de visite and the cabinet form — ^first adopted in England, and a great favourite in America — are not confined to portraits, but also employed for landscapes and photographs taken from oil paintings. Millions of these pictures are sold every year, and a properly arranged album for preserving them is found in almost every family. Photography admits of such small forms because of its fine details, but it is by no means confined to them. It freely admits surfaces that take in a portrait of life-size. The production of the latter necessitates a peculiar process, called the enlarging process, which wlLI be treated of in a future page. CHAPTEE Vn. LIGHT AS A CHEMICALLY-OPERATIVE AGENT. Theory of Photography— Nature of Light— Undulatory Theory— Semi. Tones— MoTildering away of Eed Sulphuret of Arsenic — Chemical Decomposition by Light— Colours and Tones — Their Vibrations — Eefraotion — ^Dispersion of Colours — The Spectrum — Spectral-Lines — Invisible Hays — Photographs of Lunar.Landsoapes — Abnormal Photographic Effect of Colours — Photography of the Invisible. Goethe says, "All theory is grey, and the golden- tree of hfe green." This saying has often been mis- understood and abused, especially by those too lazy to think ; but, faithful to its true meaning, we have first treated of a multitude of facts from life — that is, from the history and practice of photography, — and now we pro- ceed to describe, by the help of science, how and why, not the golden but the silver tree of photography becomes verdant, blooms and bears such splendid fruit. Two sciences join hand to accomphsh the wonders of photography. One is Optics, a division of Physics, and the other Chemistry. We have already shown that they alone are inadequate to fulfil the requirements for the production of a photograph, ^sthetical claims have to be considered ; and thus photography unites in itself the provinces of natural science and of the fine arts 66 THE CHEMISTET OF LIGHT. ■whicli seem remote and incapable of union. We shall attend first to the optical principles — that is, to light — as the force which occasions the chemical changes in photography. We shall see that its chemical operations have not only become the basis of our art, but that they have played, and still play, a still more important part ia the development of our planet. We are aware of the existence of sun, moon, and planets. We know their distance ; nay more, we know their elements, though we are separated from them by millions of miles. We are indebted for all this knowledge to light. What is light ? An undulation of the ether. And what is the ether? An infinitely delicate fluid, which fills all the space of the universe, and undulates like all fluid. If we throw a stone into water, waves are produced — ^that is, circles or rings, or hills and valleys, are formed ; these appear to widen out from a centre, and as they extend become gradually less, until' they finally disappear. If several Httle stones are thrown at the same time iato the water, each "of them forms its own system of undula- tions. These intersect each other in the most compH- cated manner ; and, although a confusion of rings takes place, it is wonderful that none of them disturbs the other, and that each circle widens out regularly from its own centre, where the stone feU into the water. (See Fig. 13.) If a handful of sand, which contains many thousand grains, is thrown into water, and if the attention be directed to the undulations of a single grain, it will be probably remarked that this one, without being affected by the countless other waves, widens out into a regular circle. LIGHT AS A CHEMICALLY-OPEEATIYE AGENT. 67 These undulations are one of the most remarkable movements in nature, taking place not only in water, but in the air, where they occasion the propagation of sound. The peculiar feature of the undulatory movement consists in the fluid appearing to advance without really doing so.. If, sitting on the side of a sheet of water, we see an undulation approach, it appears exactly as if the particles of water were approaching us from the origin of the movement. It is easy to prove that this is an error by throwing sawdust or a piece of wood into the water. It dances up and down upon the ripples with- out moving from the spot. Indeed, the undulation is itself only an up and down motion of the particles of the water, and this movement it ^'S- ^^■ communicates further and further to the neighbouring particles of water. Exactly in the same manner' light spreads in undula- tions from a luminous body through the ether of space in all directions. The movement of the undulation we call a ray of Hght. We perceive it as soon as it reaches our eye, whilst the vibrating ether strikes our retina. Now, we know that the undulations of tone are able to set other bodies in motion. If the A or second string of a violin is struck, the A string of a piano standing near sounds distinctly with it. Nay, even if the damper of a piano is raised and any tone made to vibrate, instantly the string of the violin sounds which has a 58 THE CHBMISTET OP LIGHT, similar tone. The same thing happens with a glass bell of the same tone. There are people even who can make a glass break by a shrill tone of their voice. The glass is so shaken by the violent undulations of the air that it falls to pieces. Under such circumstances, it need not smrprise us that the undulations of light agitate bodies so forcibly that they fall to pieces. Eed sulphuret of arsenic offers the most remarkable example of this kind. This is a beautiful mineral of a ruby red colour, in the form of splendid crystals, which consist of sulphur and arsenic. If a crystal of this kind be exposed for months to the light, it becomes pliant and falls into powder; and in this way many very fine pieces of this beautiful mineral have been lost in the miner alogical museum of Berlin. This is only a mechanical, and not a chemical, opera- tion of light ; but it gives an insight into its chemical working. Heat occasions chemical decomposition by extending bodies, and thereby removing their atoms so far apart that the chemical power which unites them loses effect, and the component parts separate. Thus the oxide of mercury is by heat resolved iato its parts, mercury and oxygen. This decomposition is effected by light when the atoms of a body are agitated by its undulations, that is to say, are made to vibrate ; and if these, vibrations are unequal, a separation of the parts takes place, and the body falls to pieces. The undulations of light are not a fiction. Not only has their existence been ascertained, but their size has been determined. The .latter is extremely minute, but nevertheless is susceptible of measurement. LIGHT A3 A CHBMICALLT-OPEEATIVB AGENT. 59 The waves of sound and the waves of light have there- fore a certain analogy ; and as there are different tones in music, so are there different tones in light. The number of tones is great. The simplest piano has nine octaves, and there are tones below and above it. But the number of colours is small ; only seven of them can be distinguished — red, orange, yellow, green, blue, dark blue, and violet, — the weU known colours of the rain- bow. The painter, indeed, contents himself with three ground tints — yellow, blue, and red. All the others are the result of their mixture ; and the larger scale of colour of the painter consists not of simple tones of colour, but of what may be called chords of colour. The deep tones of music give few undulations, the higher tones more. For example, an A string makes 420 vibrations in a second, the A an octave lower makes 210, the great A 105. In light, red is the colour which gives the fewest vibrations ; it is the lowest tone in colours, and violet is the highest, giving vibrations twice as rapid as red. With regard to tones, we know that they all spread with equal rapidity in the air ; if this were not the case, a piece of music would sound in the distance as the most disagree- able discord. It is the sanae in the kingdom ol light — the colours, without exception, are propagated through the ether with equal rapidity, the red as fast as the violet. But, whilst the reverberation in the second passes over only 1024 feet = 333 meters, in the same time the light hastens 42,000 'miles, and the deepest tone of colour — red — ^traverses in a second 420 billion of vibrations ; that is to say, a million times million as many as the tone 60 THE CHEMISTET OF LIGHT. ■which is marked in music with a bar over the a,* that is, i 122= The small number of the colour-tones compared with the large number of musical tones is very striking. But the fact is, that, besides the seven invisible colours, there exist invisible shades, which lie partly above and partly below the visible colours. These invisible colour-tones are partly disclosed by the thermometer, which reveals the lower tones, and partly by substances sensitive to light. For it is remark- able that the colour-tones, which are higher than the \'iolet, though invisible, have a powerful chemical effect. We name the invisible tones of colour above violet, ultra-violet, and those beyond red, ultra-red. In the common white light all the tones of colour are found together, and in combination they excite the feeling of whiteness ; but if we wish to consider the tones of colour separately, we must part them, and this is done by the help of a prism. Every polished crown-glass prism causes the rays seen through it to appear like a rainbow streak con- taining the primitive colours we have named above. This separation of the colours in the prism takes place by refraction. If a ray of light passes from one transparent body to another, it is deflected from its rectilinear direction, and this deflection is named refraction. * We may here remark that the tone a is not everywhere the same. The a of the Berlin Opera is the highest ; it has 437 vibrations, — the Italian Opera at Paris only, on the contrary, 424 vibrations. We have adopted for the sake of simplicity a ronnd number, 420. LIGHT AS A CHEMICALLT-OPEEATIVE AGENT. 61 For example, if the ray a n (Fig. 14) strikes a ■watery- surface, it does not continue in its original direction a n, but in the direction n b. If at the point n, where the ray falls into the water, a perpendicular hne n d he raised, this is the plumh line ; and the rule is, that if a ray passes from a thinner medium (for example*, air) into a thicker one, it approaches the plumb line, for « 6 is evidently nearer to the plumb line than n a. It is otherwise if a ray passes from a denser to a thinner medium, — for instance, from glass into air, — ^then the ray n b departs from the plumb line n d ; that is, the angle which it makes with the plumb line after refraction is greater than the angle which it makes with it before. Now, it is a remarkable fact that the light of unequal shades of tone is refracted also unequally. If a bundle of white sun's rays is suffered to faU on a piece of glass, the violet rays are deflected in a greater degree than the blue rays, the blue more than the green, yellow, and red ; and the result of this is that the white bundle of rays is decomposed into a rainbow - coloured fan, violet, ■^'S- !*• iadigo, blue, green, yellow, orange, and red. This phenomenon is the cause of the rainbow. If a ray a falls on a drop of water (Fig. 15), it is refracted and at the same time divided into a coloured fan, which is reflected from the lower part of the drop, suffers again a refraction and dispersion of colour h, and issues as a broad bundle of colour. In open daylight this cannot be clearly seen, because our eyes are dazzled by the 62 THE CHEMISTKT OP LIGHT. Fig. 15. clear light surrounding them. In order to observe the pure colours of the spectrum, it is best to place it in a darkened room, in which the light is allowed to enter only through a small slit (b Fig. 16). The prism s is placed on a line in front of the chink, when the colours of the spectrum are clearly seen upon the opposite wall. If the chink is sufficiently narrow, a row of dark lines is observed within it, which cut through the coloured stripes perpendicularly. These lines were first seen by Wollaston, studied more exactly by the celebrated Frauenhofer, and called after him Frauenhofer's lines. The lines are al- ■ ways found on the ^M same spot, so that ^M they can be con- H sidered as' natural ^M music lines, upon IB which the scale of |l colour is written ; IB and as the music ^ lines serve for the recognition of the musical tones, so do the lines of the spectrum indicate the exact places of the scale of colour. If we use the term green in the spectrum, this would be a very vague designation ; whereas by presenting the line on the spectrum in which green is found, its place Pig. 16. LIGHT AS A CHEMICALLT-OPEEATrFE AGENT. 63 is at once made known. To this end certain character- istic names 'were given to the lines by Frauenhofer, which he indicated by letters ; a certain line in the red he called A, another in the yellow D, one in the violet H, and H'. As the number of lines reaches several thousand, these letters do not suffice to indicate them aU. (See Fig. 17.) The lines thus named are found especially in the sun- light ; the light of other stars commonly shows other lines. The hght from artificial sources does not show dark, but bright lines ; a flame coloured yellow with kitchen salt shows, for example, a very characteristic line in the yellow ; a burning magnesium wire shows more blue and green lines. The situation of these lines agrees exactly with that of certain dark lines in the spectrum. For example, the yeUow line in a flame coloured with kitchen salt exactly coincides with line D in the spectrum. The green lines in a flame of magnesium coincide exactly with lines S C in the spectrum. A B Q D Ml) ¥ G n Fig. 17. This remarkabl.e coincidence led to the surmise that the lines in the sun's solar spectrum might owe their existence to the same substances that produced the coiaciding lines in earthly flames. Kirchhoff converted this surmise into a certainty, and was thus able to determine from the lines in the solar spectrum the sub- 64 THE CHEHISTEY OP LIGHT. stances present in tlie red-hot body of the sun, and thus ,to demonstrate the chemical composition of a star dis- tant more than 90 millions of miles (spectrum analysis). But the spectrum contains stiU other numbers, ■which have not been discerned by the human eye, but by the photographic plate. If a sensitized plate be exposed to the operation of the spectrum, it is observed that red and yellow make only a very feeble impression upon it. Light blue pro- duces more effect, but dark indigo and violet the most ; and in the space where no rays can be perceived by our eyes, a distinct impression is produced, and extends beyond violet for a space almost as long as the visible part of the spectrum. From this fact the existence of the ultra-violet rays was ascertained. Accordingly, the retina of our eye and the photographic plate show an entirely different sus- ceptibility. Our eye is affected most powerfully by yellow and green. These colours appear to us the clearest, while the photographic plate is not at all affected by them ; but it receives powerful impressions from indigo and violet rays, which appear dark to our eye, and even from rays which to our eyes are invisible. Therefore it is natural that photography should represent many objects in a false light. Further back we called attention to the fact that photography is much less sensitive to feebly lighted objects (objects in shadow). This is most clearly seen in the fact that the eye can easily perceive objects in the moonlight, which is 200,000 times weaker than that of the sun ; whereas the photo- graphic plate of a lunar landscape is not able to produce LIGHT AS A CHEMICALLY-OPERATIVE AGENT. 65 any picture at all. The photographic lunar landscapes SQmetimes offered for sale have been taken in the day- light a^d copied very darkly, so that they produce the effect of moonlight. These pictures are very popular at Venice. This small susceptibility of the photographic plate to feeble light explains the reason vyhy shadows in photo- graphy are generally too dark. To these defects must be added the false working of light, — blue generally works clear, yellow and red work Uke black. The yellow freckles appear therefore in a picture as black spots, and a blue coat becomes perfectly white. Dark blue flowers on a light yellow ground produce, in photo- graphy, light flowers on a dark ground. Eed and also fair golden hair become black. Even a very slight yellow shade has an unfavourable effect. A photograph from a drawing is often blemished by little ironmould specks in the paper invisible to the eye. These specks frequently appear as black points. There are faces with little yellow specks that do not strike the eye, but which come out very dark in photography. A few years ago a lady was photographed at Berlin, whose face had never presented specks in photography. To the surprise of the photographer, on taking her portrait specks appeared that were invisible in the original. A day later the lady sickened of the small-pox, and the specks at first invisible to the eye, became then quite apparent. Photo- graphy in this case had detected before the human eye the pock-marks very feebly tinged yellow. In the photographs of paintings, such abnormal work- ings of colour are still more evident, and can only be removed by appropriate negative retouches. 66 THE CHEMISTRY OP LIGHT. It is proper to observe, however, tliat by no means all shades of blue become light in photography. Eor example, iadigo forms an exception, appearing as dark as in nature, and this is shown in the photographs of the uniforms of Prussian soldiers. The reason of this is, that indigo contains a considerable amount of red. On the other hand, cobalt blue and ultramarine produce almost the effect of white. Again, cinnabar red works dark, also English red ; whereas madder red, which contains blue, becomes very light. Chrome yeUow becomes much lighter than Naples yellow, Schweinfurt's green becomes lighter than cinnabar green. No one of our pigments contains a perfectly pure spectrum colour, but consists always of a mixture of different colours, and therefore is essentially modified by photographic operations. If the effect of the colours of the spectrum on photo- graphic plates is more narrowly examined, it is observed that indigo produces the greatest impression. Never- theless, the differently sensitized photographic prepara- tions offer somewhat different results in this respect. Chloride of silver is most sensitive to violet, but non- sensitive to blue. Bromide of silver is also sensitive to green, and iodide of silver only to violet and indigo. Mixtures of iodide and bromide of silver are only sensitive to blue and green. The writer of this work succeeded, in the end of 1873, in making photographic plates sensitive even to those colom-s that were before considered to be inoperative, i.e. yellow, orange, and red. He found that if certain coloured substances that absorb light were added to bromide of silver, which is by itself too little sensitive to green, the sensitiveness of this bromide to green is considerably increased. In like manner, if LIGHT AS A CHEMICALLY-OPEKATIYE AGENT. 67 coloured substances absorbing yellow or red ligbt are added to it, they make bromide of silver sensitive to yellow and red light. After this discovery, we may hope that the difficulties attending the taking of coloured objects may be soon overcome. Mention has often been made of the photography of the invisible. The cases already recorded of the photo- graphs of invisible pock-marks belong to this. But the photography of an invisible quinine writing is especially understood by the term photography of the invisible. If a writing is made on paper with a concentrated solution of sulphate of quinine, the result is scarcely visible. If this is photographed, it appears black and plainly visible in the picture. The sulphate of quinine has the property of lowering the tone of violet, of ultra- violet and blue rays ; that is, of converting them into rays of less refraction and of less chemical effect ; there- fore the light issuing from quinine produces little or no effect, and the written characters become black. This property of the sulphate of quinine serves also to make ultra-violet rays visible. If a piece of paper that has been rubbed with sulphate of quinine is held in the spectrum, the originally invisible ultra-violet part of the spectrum is seen to shine in the bluish green light. Other substances produce this effect, such as uranite, Devonshire spar (fluor), and therefore this property has received the name of fluorescence. 68 IHE CHEMISTRY OP LIGHT. CHAPTEE VIII. CHEMICAL EFFECT OF DIFFERENT SOtTECES OF LIGHT. Artificial Light— Magnesium Light — Lime Light — Electric Light — Eepresentation of Subterranean Places by Eefleoted Sunlight — Chemical Intensity of Sunlight and of the Bine Sky Light — Breath, ing of Plants nnder the Influence of Light —Effect of Light in the History of the Development of the Earth and in the Economy of Nature. From the facts explained in the foregoing chapter, it follows that chemical effects are chiefly produced by the ultra-Tiolet, violet, and blue rays. It is therefore evident that a light, from whatever source, will produce chemical effects with an intensity proportioned to the amount of- these rays it contains. Lamplight, gas and petroleum light are very poor in such rays. Therefore these operate only feebly on the photographic plate, and photographers can prepare their sensitive plates in a subdued lamplight. This is also done frequently in the day by allowing the light to pass through yellow glass. The white Bengal light of arsenic, the flames of the blue Bengal light, and those of burning sulphur, produce a much more powerful chemical effect. The latter possesses only a small illuminating power, because it CHEMICAL EFFECT OF DIFFERENT SOURCifS OF LIGHT. 69 contains yello-vsr and red rays, emitting little light ; but, on the other hand, it is rich in blue and yiolet. Photo- graphs have been actually taken by help of these flames. But the above are greatly surpassed by the effect of the lame, the magnesium, and electric lights. The mag- nesium light is very simply produced by the burning of magnesium wire. Fig. 18. Magnesium is a metal which forms the chief com- ponent part of magnesia. Magnesia is nothing but magnesium rust ; that is, a combination of magnesium . with oxygen. If magnesium wire is burned, it combines at a red heat with the oxygen of the air, precipitating the oxide of magnesium. The magnesium light is very convenient in its application. An ounce of magnesium wire. 70 THE CHEMISTRY OF LIGHT. sufficient for fifteen to thirty experiments, can be easily carried in the pocket. But the general use of the light is impeded by the price of the metal (five groschen the gramme *) and by the smoke which it emits. The ■writer of this book has repeatedly employed it with success in taking the sculptures in the sepulchral monu- ments of Egypt. When burning the magnesium wire, Solomon's lamp is used (Fig. 18). This consists of a roimd vessel K, upon which the wire is coiled, a watch- work O, which conducts the wires by means of cyhnders through the pipe R, at the top / of which the wire is lighted. The apparatus, by the concave mirror 0, throws back the light as a parallel bundle of rays. Fig. 19. By means of the handle H, the lamp with its bundle of rays can be turned in any direction, and the watch- work can be instantly stopped by the key. The magnesium Hght is surpassed in strength by Drummond's lime light. This is produced by a gas or spirit flame, into which oxygen gas is blown. The oxygen gas is produced by a salt rich in that element, * 1 grosoheu = siz-fif ths of a penny, or lid. nearly. 1 gramme = the lOOOtli part of a cubic metre, about nine solid feet of water at the ordinary average temperature. CHEMICAL EFFECT OF DIFFERENT SOXJECES OF LIGHT. 71 the chlorate of potassium. This salt contains the oxygen combined with a solid. Being heated, it escapes as a gas, and is received into an india-rubber bag. (See Fig. 19.) This bag is closed by means of a stopcock, and when used is placed between two pieces of wood 6 &, a weight being placed on the upper one. By the pressure of this weight the oxygen gas pours through, the cock h, and the india-rubber pipe n, into the oxygen lamp D. To this is attached a burner H F, running into the point I. The lighting gas which serves for combustion enters through the cock L, which is connected with a gas .tube. The combustion takes place at the poiut I. Without oxygen the lighting gas burns with a clear but soot- producing flame ; but as soon as the bxygen is turned on, the flame becomes smaller and blue in . colour, and burns with an iatense heat. Its illuminating power is small, but as soon as the flame has brought the lime cylinder to a red heat, a dazzling white light issues from it, which has a very intense effect in photography, and has been used with success by Monckhoven and Hamecker to produce pictures on an enlarged' scale. The same apparatus serves for the production of what are called cloud pictures. The electric light, produced by help of an electric battery, has a still more powerful effect than the lime light. If a piece of cannel coal (fe Pig. 20) and a piece of zinc are dipped together into an acid (diluted nitric acid or sulphuric acid), electricity is developed, which produces 72 THE CHEMISTRY OF LIGHT. a spark on bringing together- the two ends of the zinc and coal rising above the fluid; this spark is, however, very feeble. But if several vessels containing zinc cyhnders z and pieces of charcoal k are employed, the spark becomes very intense; and, as we are able to increase to any extent the number of these elements, we are able to produce a cone of light of any degree of brilliancy, exceeding all other artificial light. In arranging electric bat- teries of this kind, the zinc of one element is connected with the charcoal of the following, and the zinc of the latter with the charcoal of the third ele- ment. (See Fig. 22.) If the two wires issuing from Z and C are brought together, a spark of "light is produced by the electric stream, burning the metal' wire. Fig. 21. Kg. 22. The Hght is generally produced between cones of charcoal placed in front of a concave mirror (Fig. 23). CHEMICAL EFFECT OF DIFFEEENT SOUECES OF LIGHT. 73 The apparatus S and S' serve to approach or withdra-w the cones, while the upper one is connected with the ■wireiT by the foot F ,- the lower one is connected with the wire Z of the electric battery. Thirty-six elements Fig. 23. similar to those of Fig. 21 suffice to produce the electric light. The arrangement of the battery makes the application 74 THE CHEMISTEY OF LIGHT. of the light inconTenient. In other respects this light surpasses all others in photographic effect. Nadar has made with it many excellent pictures in the catacombs of Paris. It has also been used to take portraits. But in the latter case, the employment of such a dazzling artificial light is attended mth the drawback of occasioning harshly defined shadows, that disfigure the portrait. It has been attempted to evade this by allowing an electric light of less power ta operate on the shaded side ; but it is difficult under this dazzling light, as in the sun- light, to prevent the contraction of the features. Fig. 24. It thus appears that aU these artificial lights are only auxiliaries to photographic purposes, especially as they are so expensive. Accordingly, their use wiU be confined to places that cannot be lighted in any other way. The writer of the present work has used sunlight with great advantage when engaged in photographing Egyptian sepulchres. He brought the light into subterranean passages by means of reflection. CHEMICAL EFFECT OF DIFFERENT SOURCES OP LIGHT. 75 ' Let the reader imagine a mirror set up in the open air, reflecting the sun's rays through the sepulchral entrance T, into the subterranean vault G. In this vault they are received by a second mirror, which throws the rays on the surface of wall W, of which a photo- graph has to be taken. I admit that nothing but a speck of light is thus received ; but if, during the time of exposing the photographic plate, this speck be allowed to move over the part of waU W, of which a photograph is to be taken, all parts of the object receive successively enough light to allow of photographic effects. The movement of the speck over the wall is effected by the agitation of mirror h. Braun of Dornach, by help of the same procedure, was able at a later date to reproduce the very dark frescoes -of Eaphael and Michael Angelo in the Sixtine Chapel and in the galleries of the Vatican, and produced ex- cellent results. The sunlight remains the most important source of light for photographic purposes. The clearness of this light is, however, exposed to great variations. Even the naked eye recognizes that the sun is much brighter at noon than in the morning and evening. According to the measurements of Bouguer, this difference is so con- siderable, that the sun at an elevation of 50*2 above the horizon is 1200 times brighter than at sunrise. The eye, moreover, perceives a decided difference of colour between the sun on the horizon and the sun at the zenith. The latter appears white, the former of a more reddish hue ; and, on making experiments with the spectrum apparatus, it is found that in the setting sun the reddish rays predominate, while the blue and violet are in part wanting. 76 THE CHEMISTEY OF LIGHT. It follows hence, that the chemical effect of the sun- light is very feeble ia the morning and the evening ; that it increases as the sun-rises ahove the horizon, and that it attains its greatest intensity about noon. The cause of the red hue of the morning and evening sun is found in the fact that the particles of air partly repel the blue rays — ^for which reason the air (that is, the sky) appears blue — whereas they admit the yellow and red rays more easily. If E (Fig. 25) is the earth surrounded by the atmo- sphere A, S the sun at moment of sunrise, S" the sun at the moment of sunset for the place 0, and S' the sun at noon, it is apparent that the sun's rays at sunrise and sunset have to travel much farther — namely, the distance between A and — than when the sun is at the Fig. 23. zenith S'. But in proportion as the stratum of atmo- sphere through which the sun must pass to arrive at the spectator, the weaker it becomes. It foUows from this that on high mountains the chemical effect of the rays of light must be more intense, and this has been proved by experiments on the Alps. CHEMICAL EFFECT OF DIFFERENT SOURCES OF LIGHT. 77 But not only are chemical effects produced by the sunlight ; the blue sky, which is nothing but reflected sunlight, is likewise operative, and powerfully so, through its blue colour. It has been already stated that the blue colour of the sky proceeds from this, that the particles of the air reflect more especially blue light. But the quantity of this reflected blue light varies with the hour of the day, being strongest when the sun is highest (that is, at noon), and it diminishes in proportion as the sun approaches the horizon. Therefore photographers are wont to" take their photographs of portraits when they only use the light, of the blue sky, at noon ; that is, between 10 a.m. and 2 p.m. During these hours the chemical effect of light remains almost the same ; after- wards it diminishes rapidly, — quicker in winter, slower in summer. Thus the chemical power of light, accord- ing to Bunsen, expressed in degrees, is at Berlin : — S o'docli. 1 o'clock. 2 o'clock. 3 o'clock, 4 o'clock. 5 o'clock. 6 o'clock, 7 o'clock. 8 o'clock. From June 21 38» 38 38 37 35 30 24 11 6 From Dec 2X 20" 18 15 9 It appears from this example how extraordinarily weak is chemical light in winter (for example, towards noon on the 21st December about half as powerful as towards noon on the 21st June) ; moreover, how small the amount of chemical light is which is diffused by the blue sky on the 21st December, on account of the short- ness of the day. Therefore photographers must expose much longer in winter than in summer, and, ' their printing process being slower, they take much longer in winter to copy the same number of pictures. 78 THE CHEMISTET OF LIGHT. Now the intensity of the blue sky light depends on the position of the sun, and the latter varies, not only according to the different seasons, but also at the very same seasons on different parts of the earth. If circles be drawn round the earth from pole to pole, ■we obtain what are called meridians (m m Fig. 26). All places that are situated on the same meridian have noon at the same time, but the height of the sun varies very much according to the distance of the place from the eqasbtor. If circles be drawn round the earth parallel to the equator, they form what is called lines of latitude. If the sun is at a particular place on the equator per- pendicular at noon, at the 10° of north latitude it is 10° lower; that is, the height of the sun (or the dis- tance of the sun from the horizon '^" ' expressed in angular measurement) is 80°. At 10° further north, the position of the sun at the same time is only 70°; and at the pole, which is 90° from the equator, the height of the sun = ; that is, the sun is on the horizon. The chemical strength of the blue sky light varies greatly, corresponding to the different positions of the sun at the same time. Thus,' for example — At Cairo, on the 21st Sept., the strength of Kght at noon = 105° At Heidelberg „ „ „ = 57° At Iceland „ „ „ = 27° Therefore, the more southerly a place is, the richer it is in the amount of light it offers to the photographer. CHEMICAL EFFECT OF DIFFERENT SOURCES OF LIGHT. 79 Accordingly, the American photographers are better off than those of Germany and England. These differences in the intensity of chemical light are yet essentially modified by the state of the weather. If the sky is covered with grey clouds, the chemical intensity of light is considerably less than with a perfectly clear sky. On the other hand, white clouds increase the chemi- cal intensity of light very decidedly. In the autumn the chemical intensity of light is much greater than in spring, perhaps in consequence of the greater transparency of the air. According to Eoscoe, it is in August and Sep- tember more than one and a half times as great. These variations in the chemical intensity of Ught are* very important to the life of plants. The green leaves of plants inhale carbonic acid and exhale oxygen under the influence of light. But this breathing process does not take place without the presence of light. The green colour of leaves and the variegated scale of colours in flowers only exist under the operation of light. In the dark, plants only develop sickly blossoms, like the well- known white germs of potatoes kept in cellars. The necessity of light for the life of plants is also seen in the effort made by plants kept in darkened rooms to reach the apertures which admit light, growing as it were towards them. Hence a plant develops with an energy.proportioned to the intensity of the hght. Ac- cordingly, the greater fruitfulness of the tropics is to be ascribed, not only to the higher temperature, but also to the greater chemical intensity of light. Eecent observa- tions have established that the yellow and red rays, and not the blue and violet, produce the greatest chemical effect on the leaves of plants. 80 THE CHEMISTEY OF LIGHT. We have now arrived at the knowledge of the import- ance of hght for the economy of nature. The atmo- spheric air consists of two kinds of gas, oxygen and nitrogen, which are in combination. Nitrogen is a perfectly innocuous kind of air, serving to attenuate the oxygen ; for the latter alone, though essential to life, would be injurious. In breathing, part of the oxygen is absorbed in the lungs : it forms, with the organic constituent parts of the body, carbonic acid and water. The carbonic acid and water are exhaled by us and dispersed again in the air. It is easy to prove by an experiment that a consider- ' able amount of carbonic acid is contained in the air we exhale. Carbonic acid forms, combined with lime-water, an insoluble precipitate called carbonate of Hme. If now we exhale through a glass tube, letting our breath pass into the perfectly clear lime-water, the latter becomes troubled by. the formation of carbonate of Hme. Hence the amount of oxygen in the atmospheric air is con- tinually diminished and converted into carbonic acid. The same result is produced on a larger scale by the process of combustion. In this process a combiriation takes place of wood or coal with oxygen, and the result is again principally carbonic acid. It might be supposed from this that, in the course of time, the amount of oxygen in the air must diminish, while that of carbonic acid would increase. This actually takes place in closed spaces. Leblanc found that, after a lecture in one of the lecture rooms of the Sorbonne at Paris, the air had lost one per cent, of its oxygen. In the open air this diminution of oxygen and increase CHEMICAL EFFECT OF DIFFEEENT SOUBCES OF LIGHT. 81 of carbonic acid gas is not noticed, and the reason of this is that the carbonic acid formed by combustion and the exhalations of animals is again decomposed by plants under the influence of light. Plants ab"sorb the carbonic acid, retaining the carbon and liberating the oxygen; by which means the latter, lost by combustion and exhalation, is made again available. There was a time when the atmosphere was much richer ia carbonic acid gas than now. When the incandescent and fluid masses that once formed our earth gradually became condensed, when the watery vapours were precipitated as seas, the atmosphere con- tained almost all the carbon of the earth after combus- tion ; that is, imited with oxygen as carbonic acid gas. The air was therefore at that time infinitely richer in carbonic acid than now. When at length the earth had cooled sufficiently for vegetation to be developed, gigantic plants shot forth from the warm ground under the ' influence of the sunlight. They flourished luxuriantly in the atmosphere rich in carbonic acid, the carbon of the carbonic acid passed over into the form of wood, and thus in the course of thousands of years it was continuously diminished. Eevolutions of the earth's surface suc- ceeded ; whole territories with their, forests were buried under sand and clay beds, and, becoming decomposed, were changed into coal. A fresh vegetation sprouted forth from the newly-formed soil, and agaia absorbed, under the influence of light, the carbonic acid of the atmosphere, to be once more engulphed by a fresh cata- clysm. Thus, the carbon from the carbonic acid of the atmosphere was stored as coal in the depths of the earth ; and thus the atmosphere, by the chemical effect of light. 82 THE CHEMISTRY OF LIGHT. became continually riclier in oxygen, until at length, after countless revolutions of the earth, it obtained that wealth of oxygen which made the existence of man possible, when he appeared at the end of the earth's development. We see, therefore, that the chemical influence of light has played an important part in the development of our planet, and it contiuues to do so in the economy of nature. CHAPTEE IX. ON THE EEPEACTION OF LIGHT. Simple Refraction — Deviation — Index of Eefraetion — Eefraction in Glass Plates — Prisms and Lenses — Production of Prisms or Images by Lenses. We have already pointed out (p. 60) that -when a ray of light passes the border of two transparent media of unequal density, a change of direction o takes place which is called refraction. If a smaU coin is placed in an opaque vessel, and the eye be kept in such a position that the edge of the vessel covers the coin, it is invisible. But if water be poured into the vessel the coin Kg. 27. becomes visible, and this takes place by the refraction which the rays experience in passing from water to air. (See Fig. 27.) The angle which the united rays make, before and after the refraction, is called the deviation. This deviation increases in proportion to the oblique- ness with which the rays faU upon the surface of the water. In order to determine exactly the degree of the refraction, let a perpendicular line be conceived to be 84 THE CHEMISTEY OP LIGHT. Fiaf. 28. erected at tlie point of immersion n of the ray n I (Fig. 28). This line is called the normal, or plumb line, and the angle i which the ray forms with this normal is called the angle of incidence, while the angle r which the refraction ray forms with the same normal is called the angle of refraction. The ratio of the magnitude of the angle of incidence to the angle of refraction is peculiar. If a circle he described, and from points a and b perpendicular hnes a d and 6/ are let fall on the normal, the result obtained is what mathematicians call the sine of an angle. Thus a d is the sine of i, and hj the sine of r. The ratio of the sine of incidence to the sine of refraction is constant.- This ratio is when light leaves air for water as 4 to 3 ; that is, the sine &/ is | times as great as sine a d, or sine a i is f times greater than sine bf. Light is still more refracted on entering glass. In this case the ratio of the sines is as 3 to 2. This ratio of the sines of the two angles is designated by the name ex- ponent of refraction, or index of re- fraction. If a ray of light n I falls upon a smooth sheet of glass, it experiences a similar refraction; it continues in the direction n n, and the angle of refraction at n on the glass becomes two-thirds of the angle of incidence. (Pig. 29.) On issuing from the other side of the sheet of glass, another refraction takes place ; but in this case the angle Fig. 29, ON THE EEFEACTION OF LIGHT. 85 of refraction at n' in the air becomes one and a half times larger than the angle on the glass, and as the angle at n is equal to the angle at n', the angle of emergence r n' is of the same magnitude as the angle of immersion n I; that is, the ray continues, after refraction, in its original direction. At all events, it only experiences a prolongation parallel with itself. Therefore we see through our windows in the same direction in which they are really situated. The ratio is entirely different when the spectator looks through a glass having three faces. If the eye is at o, and an object at a, and a prism with three faces be held close to the eye, the object is not seen at a, but in the direction of a'. a d suffers a deviation at the first face of the glass, taking the direction d c; ai the refraction on the second face it makes another, o c. Both deviations correspond. Fig. 30. The incident ray Kg. 31. The greater the magnitude of the angle x which the two faces of the prism, through which the ray passes, make with each other, the greater is this deviation. bb THE CHEMISTRY OF LIGHT. Thus the deviation at prism d is greater than at prism c, and at prism a it is greater than at prism b ; because the angle of refraction a; in 6 is greater than in c, and at a it is greater than in b. If a glass structure be erected, consisting of separate prisms of different angles, and if a bundle of parallel rays be conceived to fall upon it, the ray a is more ! ». ! _l01CMirHOI3fl*PKED BY S.H PI Pjage254. CHROMO-PHOTOaEAPHY. 255 blemish ; yellow spots, scarcely visible to tbe eye, have the same effect in photography as black. We knew a case in Kron's photographic printing-office where an unblemished drawing of a map came out as a photo- graph full of spots. The defect was attributed to the chemicals, until it was found that minute rust spots in the paper, which had got into it during manufacture, were the cause of the defect. In such cases the evil can only be remedied by suitable negative retouche. The nature of photo-zincography will now be clear to the reader : as the zinc plate is so like the stone, the treatment is the same. The negative is either copied direct on the zinc plate, coated with gelatine and chromium, or a copy from the negative is prepared on chromo-glucose paper; the paper is inked and trans- ferred to the zinc plate, being pressed together with it. After this the zinc plate can give impressions. It must be remarked in this connection, that even without photography, direct mechanical copies can be made of maps, writings, etc., if the original be executed in oily or analogous colours. This takes place by means of the anastatic process. This process is based on moistening the original on the reverse side of the draw- ing with acidulated gum-water, and then damping it from above with a fresh colour ; this only adheres to the oily strokes of the drawing or printing. The original, thus freshly inked, is then placed on a fresh stone, or a freshly cleaned zinc plate, and put under a pressm-e. Then the drawing passes over to the stone or the zinc, and can be easily multiplied by roUing and printing. It is difficult to preserve the original, which suffers greatly under pressure. StiU more difficult is it to reproduce a 256 THE CHEMISTRY OF LIGHT. pure stroke, for these are often extended widely by the pressm-e, and if the strokes are too thin they run together, as in mountain lines in maps ; therefore the process has been more applied to copy antiquarian books, which have been reproduced page by page in this way. It is self-evident that only reproductions of the original size can be made by the anastatic process. We have to mention another process of photo-litho- graphy, based on the application of asphalt. We have already described this in our first chapter as a sensitive substance, and also a process called heliography, which produces, by means of photography, copper plates and steel plates for printing. Asphalt serves also for photo- lithography. A lithographic stone is sprinkled with a solution of asphalt in ether, allowed to dry in the dark, and exposed under a negative. The asphalt becomes insoluble on the exposed places, and is retained upon treating the stone with ether or benzine. If the stone is then damped, the moisture only penetrates where no asphalt covers the stone. On rolling it after this with oily ink, this is rejected from the damp places, and only adheres to the asphalt — that is, to the picture ; thus a stone giving impressions is obtaiued. This method gives good results in the hands of several practitioners, and is preferred by many to the chromium process, though asphalt is much less sensitive than chromium. CHKOMO-PHOTOGEAPHT. 257 Section IX. — PTKO-PnoTOGKiPHT with Salts of Cheomium. Poitevin's Process— Effect of Chromate of Potash on Sticky Substances — Pictures Developed by Dust— Pictures on Porcelain — Oidtmann's Pyro-pliotography — Application to the Decoration of Glass — Photo- graphy and Paiating on Glass. Photography has become allied to almost all the multiplying and descriptive arts, though it was at first looked upon as their rival. It is not surprising, there- fore, that it has become a help in porcelain painting and decoration. We have already seen (p. 212) the peculiar process of changing silver pictures into gold and platinum pictures, transferring them to porcelain, and burning them in. That method might be called a moist process ; the same end can be obtained by a dry method, and by the help of salts of chromium. This original method has also b^en invented by Poitevin, and subsequently was materially improved by Joubert in London, and Ober- netter at Munich. It consists in this : that a mixtm*e of gum, honey, and chromate of potash is pom-ed on glass ; the film is carefully dried in the dark, and then exposed under a positive. The film of gum is freshly prepared, sticky, and holds fast the scattered coloured powder, but when the film is exposed, it loses its sticki- ness. If this exposure takes place under a drawing with black strokes, the film under them will retain its stickiness, but lose it beneath the white, transparent parts of the paper. Therefore, if the fihn, after exposure, be powdered ia the dark with any colour in powder, this adheres where the strokes of the drawing have protected the film, but not at other places, and thus a picture in powdered 258 THE CHEMISTRY OF LIGHT. colour is obtained. If this coloured powder and its under- layer is fire-proof — as glass and porcelain — ^the picture obtained can be burnt into it, and pictures of very- different shades can be produced, according to the choice of the powdered colour. Pictures of this kind can be transferred from one under-layer to another. If a coUodion film is poured upon the powdered picture, if this is suffered to dry and then the whole thrown into water, when there the collodion film with the picture can be easily taken off, stuck on, and burnt iato other surfaces — glasses, cups, etc. Thus Joubert in London has actually burnt in large pictures on glass. Obernetter at Munich, and Leth at Vienna, Leisner at Waldenburg, and Stender at Lamspringe, Greiaer at Apolda, and Lafon de Camarsac at Paris have produced encaustic pictures on porcelain in the same manner. This process is not much employed for portraits. On the other hand, Oidtmann at Linnich,* has employed it advantageously in glass manufacture. He has copied patterns of carpets from lithographs directly on glass, and burnt them in, thereby producing cheap window ornaments, which can be painted and embellished by the hand. At the Vienna exhibition, there was over the door of the Emperor of Germany's pavilion a rosette ten feet in diameter, produced by Dr. Oidtmann on the above system. The same person has also employed the process to produce mosaic glass pictures, similar to the mediaeval glass paintings on glass. These mosaic glass pictures are produced by cutting out colom-ed pieces of glass corresponding to the figures and their * See " PhotograpUsohe Mittheilnngen," Jahrg. 1869. Berlin : Oppenheim. CHEOMO-PHOTOGKAPHT. 259 colours. For example, for a human figure, the outline of the face was drawn on a flesh-coloured glass slab and cut out ; the same thing took place for the drapery, on glass slabs corresponding to the colours. The lights and shades and details — for example, nose, mouth, and eyes — were then drawn with black moist colour on the proper piece of glass assigned to it, and Isurnt in, after which all the separate pieces of the glass were soldered (or cemented) together. Dr. Oidtmann does, by means of photography, what the draughtsman does in this mosaic glass-painting. He copies the outlines of the face from the large-sized original photograph, or the original woodcut, on the proper piece of glass, and powders it with moist black paint, and he thus obtains a picture that can be burnt in, and which can be treated in the manner described. At the Vienna exhibition there was a copy of " The Crucifixion " by Diirer, produced ia this manner, and composed of 150 glass pieces. Dr. Oidtmann prepares the large-sized original pictures by magnifying little woodcuts according to the photo- graphic manner (see p. 95). Di. Oidtmann has also attempted to produce pyro-photographs, by proceeding on the principle of chrqmo-lithography (see p. 250, Photo-lithography). He copied the similarly coloured parts of a painted drawing — covering over the others — on a film of gum and chromium, powdered this with a suitable colour, and then copied the other colours of the original successively in the same manner. He thus obtained a powdered picture, which was then burnt in. 260 THE CHEMISXEY OF LIGHT. Section X. — Photogbapht and the Sand-blotping Peo( The Nature of the Sand-blowing Process — Its Connection wim tho Pigment Press— Its Employment in Heliography instead of Corrosive Acids. Tilghmann, at Philadelphia, made the ohservation, during his residence at the watering-place Longhranche, that the windows exposed to sea wind became quickly- misty. He found that this was occasioned by fine sand, which the wind drove against the window; this gave him the idea of making ground glass by sand blown on to it, and this succeeded perfectly. He covered a glass surface with an iron mould, in which figures and letters were cut ; he kept this in a current of air bringing sand with it. In a short time this made ground glass at the places that were exposed, and Tilghmann obtained thus a drawing of the incised figures. A blast of only four inches hydraulic pressure and a period of ten minutes are required for this work. If the air pressure is stronger, or steam is used instead, conveying sand, and having a pressure of 60 to 120 lbs. to the square inch, the effect is immense. Sand blown with such power through a narrow pipe bores deep holes into the hardest stones, and even into glass. The process has been used to bore stone and metal plates. If a mould of cast-iron is placed on it in which the figures have b^en cut, they can be deeply engraved in a short time in the stone. The iron plate is, no doubt, injured, but much more slowly than the stone slab. A cast-iron plate ^g of an inch thick is only reduced -^^ of an inch, whilst a section 300 times deeper is made in marble. India-rubber endures the sand stream almost as weU as iron. You might cut into marble with an india rubber-mould 200 CHEOMO-PHOTOGEAPHT. 261 times as deep as the depth of the mould, without much injuring it. With the pressure of 100 lbs., such a sand stream can penetrate IJ inches deep into granite, 4 inches into marble, and 10 inches into soft sandstone. The circumstance that soft bodies act as shields in this has led to elegant applications of this method in the industrial arts. For example, if glass be covered with lace pattern and a sand stream take effect upon it, the glass becomes ground in the meshes, and a copy of the lace is obtained on glass. In the same manner, you can paint with gum colour upon glass, and this drawing can b3 produced . clear on an unpolished ground by the sand blast. This circumstance led immediately to the application of photography. If a pigment impression (p. 239)— that is, a chromo-glucose picture — ^is produced on glass by transferring a prepared impression directly on glass (see above), the glass surfaces at aU the places of the picture are protected by a layer of gelatine. If now a sand stream is allowed to operate upon it, it polishes the glass only at the uncovered places ; thus an opaque and transparent glass picture is the result. If the gela- tine picture is a negative, the shadows are dim ; and such an unpolished slab is also fit for giving impressions by means of printer's ink. Corroding with acids often eats into the fine strokes and makes them broader. In place of them the heliographic metal plates of Talbot (p. 225) can be blown upon with sand — ^which, owing to its per- pendicular position, only works downwards — and thus cavities of great depth can be produced, so that plates thus blown upon can be used for high relief-printing, that is, book printing. Tilghmann recommends that a 262 THE CHEMISTRY OF LIGHT. positive of gelatine and chromium should be produced upon a cake of resin ; that this should be blown upon and deeply hollowed out ; then a form is obtained which can be first cast in gypsum, and then in metal type, which can be used for printing. These are interesting experiments, which ought in time to lead to important practical results. SEoiioif XI. — The Photometee ron Oheomo-Phoiogeapht. In many of the above described chromic processes, e.g. the production of relief-prints, pigment-prints, light- prints, etc., it is very important to determine the exact time for exposure. This is not easy, because the picture Fig. 94 appears only faint or not at all, as in pigment-printing ; therefore the state of the picture gives no safe criterion respecting the completion of the picture. This circum- stance has necessitated the application of a photometer, easily determining the duration of the exposure. Such photometers have been provided by Byng and Swann in England, and by the author. The author's photometer consists of a semi-transparent paper scale L, whose transparency diminishes from 2 to 25. (See Fig. 94.) This scale is formed of layers of paper, whose entire CHEOMO-PHOTOGBAPHT. 263 quantity is expressed by the figure printed on them ; under this scale is exposed a strip of chromic paper; that is, paper which has been plunged into chromate of potassium. The strip is enclosed in a little box in such wise that, when the cover D is shut down with the scale, the chromic paper and the scale are in close contact; the light now shines through the scale and browns the paper strips lying under it. This colouring affects, first, the thin transparent part of the scale, and passes thence to the opaque end, the rapidity depending on the strength , of the light. To know how far the effect of the light has extended, figures are printed on the scale which do not permit the light to pass ; therefore these remain clear on a brown ground, and the place to which the effect of light has advanced is perceived by the figure that appears there. To use this instrument, some experimental copies must be made first. Supposing it were desired to prepare a pigment impression from a negative, the film of pig- ment is exposed under the negative at the same time with the photometer. After some time, -lamplight is used to see how far the photometer paper is browned. The significant number is noted — ^photometer degree — and the negative is only half covered, the other half continuing to be copied until a higher degree of the photometer. Then the pigment picture is developed, and the degree of the photometer is determined where the favourable result has been obtained. Earely more than one attempt has to be made; when this has determined the degree up to which the negative must be copied, the time of exposure can always be regulated with the help of the photometer. Practised hands only 264 THE CHEMISTBY OF LIGHT. determine the degree with some negatives, and easily ascertain up to what degree a fresh negative must be copied. Section XII. — The Chemical ErrscT op Light and the Pea-Sausagb. In the campaign of 1870, the well-known pea-sausage was one of the most important articles of food for the army, and was prepared daily in many thousands of skins. The fabrication of the interior portion caused Httle difficulty, but the obtaining so many skins created much difficulty. As the supply fell short, a substitute was sought in parchment. This paper, which is produced by dipping for a second blotting-paper in sulphm-ic acid, then wash- ing and drying it, is distinguished by its skin-like properties of resistance. It is impenetrable to water, and difficult to tear. It is therefore used for the pro- duction of cheques on the Treasury. It was attempted to fabricate sausage skins with this paper, by doubling a sheet cylindrieaUy and pasting it together. No glue or gum can resist the effect of the boiling water in which the sausage has to be cooked, and so the- artistic sausage skin feU asunder. Dr. Jacobson solved the problem by producing an adhesive substance, with the help of the chemical effects of Kght, which could resist boiling water. He mixed the gelatine intended for the pea- sausage skin with chromate of potash, and exposed the adhesive parts to the light. This occasioned the insolu- bility of the gummy substance, and now the artificial skin endured boiling water thoroughly well. The number of sausage skins prepared in this way, by the chemical operation of light, amounted to many hundred thousands. CHAPTEE XVI. lEON, tJEANIUM, AND COPPER PHOTOGRAPHY. Historical — Combinations of Iron — Effect of Ether on a Solution of Chloride of Iron — Chloride of Iron and Paper — Iron Pictures in Blue — Iron.gold Pictures — Poms Process with Salts of Iron — Iodide Kctures — Combinations of TJranium— Uranium Pictures — Their Development — Copper Pictures of Obemetter. "We remarked further back that the number of sensitive substances is much greater than appears, and a close analysis was to determine that aU bodies were more or less sensitive to light. Even in the first period of photography, 1840, Herschel observed the sensitiveness of salts of iron, Burnett that of salts of m-anium, and Kratochvila prepared successful daguerreotypes on copper plates, in a manner analogous to those on silver plates. This process has been energetically cultivated, but hitherto without any important result. It has long been known that chloride of iron, a yellow substance consisting of iron and chlorine, when dis- solved in ether bleaches in the light and becomes the hypo-chloride of iron, having less chlorine. The same thing takes place in connection with paper. If clean paper is saturated with a solution of chloride of iron in six parts of water, dried in the dark, and exposed under 266 THE CHEMISTET OF LIGHT, a negative picture, the paper, which is yellow at first, becomes white under the transparent places, because the yellow chloride of iron passes into white hypo- chloride of iron. This pale hypo-chloride picture can be easily coloured intensely dark. If the pale picture is plunged in a solution of red prussiate of potash, this, ^combined with the hypo-chloride of iron reduced by light, easily produces Berlin or Prussian blue, while it leaves the chloride of iron unchanged ; in this manner a blue picture is obtained. K a pale iron picture be plunged in a solution of gold, it becomes of a light blue colour, because the hypo-chloride of iron produces a pre- cipitate of metal gold. In this manner a dark precipitate, which is produced by hypo-chloride in many substances, win give a dark colour in all such cases. Another process is the transforming of iron into iodide pictures. A piece of paper, saturated with chloride of iron, is copied under a positive (for example, a drawing). The copy comes out as a yellow drawing of an unchanged chloride of iron on a white ground. If the paper be now plunged in a solution of iodide of calcium and starch, the iodide becomes liberated, and forms with the starch a dark blue iodide starch, which gives a strong dark shade to the lines that were originally pale (Herschel). There are several other processes to make iron pictures of a darker colour. The pictures in Berlin blue do not hold, because that colour turns pale in the sun (accordingly, blue parallels rapidly lose their colour in the light). The same remark applies to pictures of iodide of starch ; the gold pictures are too pale and their restoration too costly. IRON, UEANIUM, AND COPPER PHOTOGRAPHY. 267 The salts of uranium present the same phenomena as these salts. Uranium itself is a rare metal whose combinations play a great part in colouring materials ; thus there is a yellow oxide of uranium, that can be burnt into porcelain, giving a dark green colour, and which, being mixed with glass, imparts to it a beautiful grass-green (Anna glass). Moreover, a chloride of uranium and a hypo-chloride of uranium are known, corresponding to the chloride of iron and the hypo- chloride of iron, and bearing much resemblance to the combinations of iron just mentioned. The most noted salt of uranium is nitrate of uranium, which is reduced to sub-nitrate of uranium by the influence of light in the presence of organic bodies — for example, paper recepta- cles. If a piece of paper is dipped in a solution of one part of this salt and five parts of water, if it be then dried in the light under a negative, a very faint, scarcely perceptible picture is obtained, consisting of • sesqui- oxide of uranium. If this is plunged in a solution of silver, or a solution of gold, it becomes suddenly visible, because the sesqui-oxide of uranium precipitates directly the gold or silver metal as a coloured powder (in the case of the silver, brown ; and in that of gold, violet). The uranium is too rare and too dear to be employed generally in photography. As can be perceived, the salts of iron and the salts of uranium are analogous to the salts of chromium, by only being sensitive to light in the presence of organic bodies. In a pure state, salts of uranium and salts of iron do not change in the light. The sensitiveness to light of salts of copper has hitherto only been studied very imperfectly. Copper 268 THE CHEMISTBT OF LIGHT. forms with chlorine a green salt, soluble in water, — chloride of copper, — which is reduced to hypo-chloride of copper in the Hght. Ohemetter took advantage of this fact, mixing chloride of copper and chloride of iron together, and saturating the paper with them. This was exposed to light under a negative, then plunged in sulpho-cyanide of potassium, and ultimately treated with red prussiate of potash. The result produced by this somewhat complicated process was a brown picture.* * See Togel, " Lehrbnch. der Photographie," p. 32. Berlin: Oppon. heim. CHAPTEE XVn. THE CHANGE OF GLASS IJNDEE THE INFLUENCE 05' LIGHT. Faraday's Observation on Manganese-glasa— Change of Mirror-glass in the Light— Almost all kinds of Glass are Sensitive to Light— GaflSeld's Experiments — Disadvantages of the Change of Glass in the Light — Explanation of the Change of Manganese-glass — Operation of Light on Topaz. The celebrated natural-philosopher Faraday made the observation that glasses painted "with manganese, and conspicuous for a peculiar flesh tint, became rapidly- brown in the Hght. This fact remained for a long time without further results. But some years later, other observations of the same kind were made. A very handsome plate of glass was exhibited in a mirror shop at Berhn. It bore the inscription "Manu- facture of Mirrors " in brass letters. After being exhibited for years, the business was broken up, and the mirror, on account of its beauty, was taken away by its proprietor, the brass letters were effaced, and the plate was cleaned. To the surprise of the proprietor, the letters remained plainly visible on the glass, notwith- standing all attempts to remove them. The surface was even abraded, but this did not produce any effect on the letters. It was found that the glass was penetrated 270 THE CHEMISTET OF LIGHT. with yellow marks, and that it remained white only at the places where the opaque letters had kept off the light. The plate of glass was afterwards cut into two halves. One half, with the word "mirror," remains in the hands of the Philosophical Collection of the Uni- versity of Berlin. Gaffield has lately made very interest- ing observations on the change of glass ia the light, and has thus determined that almost all kinds of glass are sensitive to the light, and that often an exposure of only a few days suffices to effect this change. Gaffield went systematically to work in his experi- ments. He cut the glass in question into two parts, placed one in the dark and the other in the light, and compared the two after a few days. In almost aU cases he remarked a darkening of the colours. Only two kinds of greenish German and Belgian window-glass remained unchanged. The glasses of a darker colour lost their colour when exposed to red heat. This change of glass in the light has a very unfavour- able effect in photographic studios. Through the yellowish colouring which their glass assumes, in time a part of the chemically operative light is absorbed in the glass. The deterioration of light thus resulting makes itself remarked in a very conspicuous manner, because the time which is necessary in order to take a portrait must be continually lengthened. The glasses that contain manganese change most strikingly. Hyper-oxide of manganese, also named brownstone, is often added to glass to discolour it. The dark green sesqui-oxide of iron in the glass is transformed, by the operation of the oxygen of the brownstone, into the paler protoxide of iron, and the dis- CHANGE or GLASS UNDER INFLUENCE OP LIGHT. 271 colouration is effected in this manner. The opposite effect takes place in the light. The oxide of iron is reduced again to sesqui-oxide of iron ; the oxygen passes to the manganese, and, forming brown oxide of man- ganese, gives rise in this manner to the dark colouring. In many minerals, light has an opposite effect to that ■which it has on glass. It does not colour them, but discolours. This happens especially with the Siberian ' topaz, which soon loses its golden-yellow colour in the light. A splendid crystal of topaz, six inches high, belonging to the Mineralogical Museum at Berlin, has in this manner lost materially in the beauty of its appearance. 272 THE CHEMISTEY OF LIGHI. CHAPTEE XVin. PHOTOGRAPHY IN NATURAL COLOURS. Observation of Seebeok and Hersohel — Beqnerel's Painted Piotnres and Silver Plates — Nifepoe's Labonrs — Effect of Black Colonrs — Coloured Pictures on Paper of Poitevin and Zencker — Want of a Pixing Medium for Coloured Photographs. Photography has already achieved grand results; but it has still one problem to solve — the production of photographs in natural colours. There are plenty of coloured photographs to be seen, but in such cases the colour has been added after by the paint brush ; it is a kind of retouche, which in most cases does not improve the picture. But we are now speaking of photographs in natural colours, reproducing the original colours of objects solely and alone by the operation of light. Numerous attempts are at hand, pointing more or less to this great end. The production of colom-ed pictures, by the chemical effect of light, ' has been successfully achieved ; but these are spoiled soon by the influence of the same agent to which they are indebted for their production. No means exist at present of fixing coloured photographs. The first attempts to make coloured pictures date a PH0T0GR4PHY IN NATTJRAIi COLOUES. 273 long way back. Professor Seebeck of Jena, as early as 1810, found that chloride of silver took in the colour spectrum almost the same colour corresponding to them. This observation, published in Goethe's " Farben- lehre," 11., page 716, passed unnoticed. Only since 1841, after the discovery of the daguerreotype, the noted Sir John Herschel made experiments in the same direction. He took paper saturated -with chloride of silver and nitrate of silver, let a powerful solar spectrum fall upon it, and obtained immediately, like Seebeck, a coloured image of the spectrum, agreeing, however, only imper- fectly with the original. Bequerel was more successful. He ascertained that the solution of nitrate of silver in Herschel's experiments had a disturbing effect, and he worked with chloride of silver alone. He employed silver plates, which he plunged in chlorine water. The plates become thus whitish, by the formation of chloride of silver, and, when exposed to the spectrum, present a picture whose colours agree very nearly with the natural ones. Bequerel observed that the duration of the operation of the chlorine water was very important, and he preferred at a later date to chlorize the plates by the operation of the galvanic current. To this end he sus- pended them to the copper pole of a galvanic battery (see p. 227) and plunged them in salt acids. The galvanic current decomposed these acids into chlorine and hydrogen. The chlorine passes to the silver plate, and forms chloride of silver. This method enables the operator to produce a film of chloride of silver of any thickness, according to the duration of the operation of the electric current. The brownish hypo-chloride of silver is thus produced, and this is chiefly sensitive to 274 THE CHEMISTE-Sr OP LIGHT. colour. Yet this sensitiveness is not great, it only suffices to fix a powerful spiectrum, but it requires a very long exposure to obtain images from the camera obscura, and all such images, unfortunately, darken through the continuous operation of light. Bequerel found that the sensitiveness was increased by heating the plates. This observation was turned to account by his successor, Niepce do St. Victor (the nephew of Nicophore Niepce, see p. 9). This savant made numer- ous experiments from 1851-67 to produce coloured photographs, and he imparted his observations to the Paris Academy. . He worked, like Bequerel, with silver plates, which he chlorized by plunging in a solution of chloride of copper and chloride of iron, then heated to a high degree, and thus obtaiaed plates which appeared ten times more sensitive than Bequerel's, and allowed him to copy in the camera obscura copper lithographs, flowers, church windows, etc. He relates that he not only obtained colours, but that gold and silver remained in their metallic splendour in pictures, and the picture of a peacock's feather retained the lustre of nature. Niepce de St. Victor introduced a further improve- ment, by covering the plate of chloride of silver with a peculiar varnish, consisting of dextrine and a solution of chloride of lead. This coating made the plates still more sensitive and durable. At the Paris exhibition of 1867, Niepce exposed the different coloured photographs that lasted above a week in a subdued daylight (they were exposed in haK-closed boxes). Among these pictures were a couple of uncoloured, but perfectly black pictures on a white ground, which PHOTOGRAPHY IN NATURAL COLOURS. 276 had been copied from copper-plate engraYings. These excited great interest, and justly so, because in these pictures the darkest influence had had most effect, and the cleanest and -whitest the least. This was, therefore, a directly contrary effect to what happens on photographic paper, where the dark produces a clear effect, and the clear a dark effect (see p. 28). This production of black by black can only be explained by assuming that the black is actually not black, but that it irradiates ultra- violet Hght invisible to the eye (see p. 60). Since Niepce, who died in 1870, the only persons who have directed attention to coloured pictures are Poitevin at Paris, Dr. Zencker * at Berlin, and Simpson in London. But the two former investigators have re- verted to the older process, as employed by Seebeck and Herschel, i.e. they prepared pictures on paper again, only the preparation of this paper was peculiar. Paper saturated with salts was made sensitive in a solution of silver, like the photographic positive paper (see p. 50), then washed to remove the solution of silver, and after- wards exposed to the light in a solution of hypo-chloride of tin. By this means violet hypo-chloride of silver is formed from the white chloride of silver. The hypo-chloride of tin only operates as a reducing medium. This paper is in itself little sensitive to colouring ; but if it be treated with a solution of chromic acid of nitre and copper vitriol, its sensitiveness increases considerably, so that it is easy to copy with it pictures of transparent colours. Nevertheless, the colours are never so vivid as in the original, the red tones showing themselves the • Those who take a speoisil interest in the matter are referred to Dr. Zenoker's " Lehrbuoh der Photoohromie." Berlin, 1868. 276 THE CHEMISTRY OF LIGHT. strongest. After copying, the pictures are washed -with ■water, to make them less sensitive to light. In this condition they showed tolerably well in a subdued light, but no means have been found hitherto to make them perfectly durable. The fixing natrium of the photo- grapher (see p. 27) cannot be employed, as it destroys the colours directly. We must hope that future investi- gators wUl succeed in supplying this , deficiency. The first attempts in uncoloured photography also failed for want of a fixing medium (see p. 6), which was only discovered seventeen years later by Herschel. CHAPTEE XIX. PHOTOGEAPHT AS A SUBJECT TO BE TAUGHT IN AE.T AND INDUSTEIAL SCHOOLS. Importance of School Photography — Its Use for Technical Institutions — Photography as an Object to be taught in Art Schools and Umrer- sities. The previous chapters prove how manifold are the applications of photography. It has entered into art, science, industry, and life as a new kind of written language. Photography is to appearance what printing is to thought. Typography multiplies what is written, photography what is drawn ; nay, more, it also draws in a chemical way. No doubt a certain technical practice of the art is required that can only be gained by experi- ence ; but it is easy to learn, and the time cannot be distant when it wiU be taught as an extension of drawing — ^itself a matter of tuition — in aU industrial schools. Years are devoted to the study of the art of drawing, of piano playing, and other things; a course of instruc- tion lasting half a year would suf6ce to learn photo- graphy. The author has for nine years presided over a pro- fessor's chair of photography in the Eoyal Industrial 13 278 THE CHEMISTBT OF LIGHT. Academy at Berlin, the only technical institution in Germany where photography has as yet obtained a footing in the curriculum. It is by no means the object of this institution to train professional photographers; it only admits photography so far as it has importance for industry and science. At this institution practical exercises are carried out in the positive and negative processes of photography, especially in its application to reproduce drawings, for taking machines and buildings ; and, moreover, instruc- tions are given on the lieht-paus process. Other technical institutions are still hesitating about adnaitting photo- graphy. The importance of the matter is still depre- ciated, and what is new is still viewed by many as inconvenient. We cannot avoid introducing, in this connection, a passage . from a work -that has recently appeared: "Photography as a Matter for Teaching in Schools of Industry," by Professor Exippendorf, of Aran. The author remarks : — *' Schools of industry are instituted with the special view to prepare pupils for the subsequent professions of a technical and industrial Hfe, and therefore they natur- ally admit in their curriculum the arts and sciences devoted to this end, especially drawing. Industrial training must see not only that these branches form an organic whole, taking the place of the old classical languages as a basis of general culture; it must also draw into the province of science the new inventions introduced in industry, and then suffer them to work back on practical pursuits in a more profitable manner. " Photography is one of the branches that has advanced most rapidly in the last ten to twenty years. It is a PHOXOGBAPHY IN AET SCHOOLS. 279 genuine product of natural science, not as the mere play- thing of accidents, but having the great merit of being first conceived as an ideal, and then practically carried out. It is therefore an art full of value in itself, based on science, and one v?hose productions delight and are gladly viewed by aU, extending the knowledge of pupils and giving an ideahzing tendency to young minds. "There is scarcely any other branch in industrial schools in which it is a downright necessity to keep in view an independent observation of the result. Physics and chemistry are taught as successful results, and give no clue to detect the source of errors. The pupil only observes what the teacher puts before him, and both are satisfied if the law is found in the experiments. A learning how to observe does not properly take place, yet this is specially fitted to sharpen the judgment. But if photography is admitted in the schoftl, we gain a branch which fixes the attention of the pupil in a way that no other can do. The study of photography is specially based on the avoidance of sources of error, and consists in the necessity of setting aside certain dis- advantages and treating their source; hence it is en- titled, when the art is properly appreciated, to be introduced in all such schools. "Many other grounds can be noticed in favour of this introduction, "Art and science are learnt in technical schools for practical ends. Practice and a knowledge of drawing are specially demanded on entering the engineering profession. It may even be affirmed, of two equally talented pupils, the best draughtsman will take the first place. Drawing is the centre of gravity for most techni- 280 THE CHEMISTET OF LIGfiT. cal professions, and for tttis reason alone it ought not to be neglected to promote technical and freehand drawing in every direction. But photography is destined to support these technical studies, as it is also a'Daode of drawing. Indeed, if it be proposed to draw a compli- cated machine — as a weaver's wheel — ^in a few minutes, photography is the only means to do this. The labour, otherwise requiring weeks, is reduced to an affair of a quarter of an hour, and is so perfectly done that all pro- portions are duly observed, and the projection must be correct from whatever point it is taken, if proper lenses be used. " If we follow ihe biography of gifted pupils, we often trace them, aided by Government stipends, going first on distant journeys to study modern and ancient monu- ments, and to bring home as faithful designs of them as possible. What a severe labour this imphes for the architect, amidst a foreign population, in a trying climate, who has to project faithful sketches in a short time amidst countless obstacles ! On the other hand, how it is aU abridged by photography! What would not the young travelling engineer give to take plans of entire manufactories which he has only a few minutes to view ? What would not the highly cultivated philologist give to retain for himself and others, in a durable form, the overpowering impressions of life in the past, which ■ he can only feel as a transient emotion, on the classical ground of Greece and Italy ? It is our duty to announce it publicly, that aU these wishes have become a possible and a tangible reality through the progress of photo- graphy, and that the practice necessary to effect this is easily attained. " PHOTOGRAPHY IN AET SCHOOLS. 281 Krippendorf omits here an important point, -which is the great value of photography to those -who are devoted to practical typography, whether it be hthography, book printing, copper-plate printing, printing of paper money, porcelain manufacture or dyeing ; for in all these branches the aid of photography is very important. In this con- nection we refer to the chapter on pyro-photography, heliography, and chromo-photography. In these branches we see photography an auxiHary to the multiplying arts. Though it has done great things in this connection, we see very few heliographers and photo-hthographers. The ground of this is found simply in the fact that art schools, training lithographers and copper-plate engravers, entirely overlook photography. It is set aside as no art at all by persons who feel themselves artists, yet to whom it would be useful. But in the before-mentioned alliance of photography with htho- graphy and metal-plate printing, good results can only be achieved by the operator being eq[ually skilled in both arts. The author has often witnessed the failure of heliographers, lithographers, and photographers who tried to work by combining the two arts. It is therefore necessary that the schools of art should take the matter in hand, and if so, a new province, hitherto unknown to the lithographer, that of light-printing (see p. 243), must soon become domiciliated in those institutions. But a knowledge of photography is equally important for painters. Photography copymg oil-paintings has taken a magnificent development in our time. Adverse opinions to it are indeed uttered by stiff art critics, such as Thansing, just as idealistic tourists formerly ranted against railways, because travel was thereby 282 a?HE CHEMISTEY 01? LIGHT. robbed of its poetry. These people were right from their point of view, hut they could not stop the intro- duction of railways ; and, though travel may have become less poetic through them, these Hnes have the advantage of giving an opportunity to persons of slender means to make excursions, and thereby to enrich their minds with a knowledge of foreign countries and people, and of improving their health. Photography affords to persons of small income similar advantages in the province of art. Paintings too expensive to be purchased by any save the rich, became slowly and imperfectly known to others by the expensive medium of copper-plate prints. These engravings were also confined to a limited circle. But now photography reproduces with the rapidity of lightning, and with all faithfulness, the latest creations of art, and thus its cheapness makes them accessible to all. The copy is not so artistic as that of the engraver, but it suffices to bring all that is new quickly to the knowledge of all, and in spite — or in consequence rather — of this, the engraving coming after stiU retains its value. Moreover, the negatives from oil-paintings require working up by retouche, in order to equalize the false effects of colouring. This retouche may be very injurious if carried out by unskilled hands. The most suitable hand is that of the painter who painted the original. Good painters have already successfully worked at reproducing negatives from their own originals, and the impressions from plates touched up by the artists them- selves must evidently have a much higher value than those emanating from other hands. This presents a fine new field for the artist ; but it can only be worked PHOTOGRAPHY IN AET SCHOOLS. 283 ■with good results if the art pupils have become familiar in art schools -with the technical routine of negative retouche, and with taking positive impressions connected with them. In conclusion we add a few words on the development of the professional photographer. We have akeady shown that if portrait and landscape photography are to produce really solid results, they require a knowledge of the principles of art. But hitherto nothing has been done to train photographers artistically. Moreover, photography can only be raised, in an artistic point of view, when art schools render a study of art possible to the photographer. The time must be at hand when aU small jealousy directed against photography must fall to the ground. Experience has already found that it is not a rival, but a handmaid to the solidly trained artist. Photography can be the more readily introduced in schools, as its tuition requires much less time than drawing, and with better results. Four hours a week for six months suffice to train a pupil enough in photo- graphy to enable him to go on by himself, even without a knowledge of chemistry. Schools of science, as well as of art, must attend to this branch, because photography is very useful as an aid to natural science. This new art gives beautiful illustrations for science and art lectures by the magic lantern. The investigator can by its means give faithful original pictures of the results of lys labours (see page 93). Hitherto proper apparatus was wanted; the magic lanterns sold in Germany gave imperfect images. Latterly, E. Talbot in 284 THE CHEMISTET OP LIGHT. Berlin, has introduced American magic lanterns, which are best adapted for lectures. The Woodbury press has been joined to the above, for the illustration of lectures ; and the latest improvements in dry plate photography have had the result that dry plates, like licht-paus paper, have become articles of trade, making the production of photographs much more easy. Thus one improvement combines vdth another to make photography what it ought to be — a universal art of writing by hght ! AMERICAN APPENDIX. Theeb is an omission in the foregoing work, which, though it may matter little with the European editions, it is worth while to point out in the American edition : the distinguished author has hardly done justice to the science of this country in its contributions to the development of his subject* Professor Vogel ascribes the first daguerreotypes to Pro- fessor Morse; and states that "his coadjutor was Professor Draper." Now, the fact is, that Dr. J. W. Draper, of New York, was the first person who took daguerreotype-portraits from the life, which was in September, 1839. He published an announcement of it in the London and Edinburgh Phil- osophical Magazine of March, 1840, and shortly afterward gave a detailed' account of the whole operation in the same journal. Dr. Draper had been already for a considerable time making researches on the chemistry of light, and was not only familiar with the whole subject, but was active in its original exploration. In his laboratory in the New York University, Professor Morse learned the art. Dr. Draper was also the first to photograph the fixed lines of Fraunhofer in the solar spectrum, and to show that they exist in large numbers beyond the" violet space. He was, besides, the first to decompose carbonic acid in the actual spectrum ; and to prove that it is the yellow light that is efficient in this change, and not the violet rays, as had been formerly supposed. APPENDIX. As has been recognized by high foreign authorities, and as is well known, Dr. Draper was the first to devise a method for measuring the intensity of the ohemical rays, by which it first became possible to subject them to quantitative investi- gations. He was, moreover, the founder of astronomical photogra- phy, having taken pictures of the moon, which were exhibited at the New York Lyceum of Natural History, March 33, 1840. Dr. Draper also established the great principle which is at the basis of spectrum analysis. In a memoir " On the Pro- duction of Light by Heat," he showed experimentally that all solid substances, and probably liquids, become incandescent at the same temperature o4 about 977° Fahr.; that the spec- trum of an incandescent solid is continuous, and contains neither bright nor dark fixed lines; that from common tem- peratures up to 977° Fahr., the rays emitted by a solid are invisible, but at that temperature they impress the eye with the sensation of red ; that as the heat of the body continuous- ly rises, other rays are added, increasing in refra,hgibility as the temperature ascends. The English professor, Roscoe, in the third and last edition of his " Spectrum Analysis," ac- knowledges that " this law was discovered by Draper ;" but the German, Kirchhofi", although he had himself built upon Draper's resylts, makes no reference to them in his historical sketch of spectrum analysis. The author of the present work touches upon these various points, but is careless of what Dr. Draper has done — an over- sight which is the more marked, as he seems careful to ac- knowledge the claims of savants in the different countries of Europe. E. L. Y. New York, April, 1875. INDEX. AccuEACY of Photographs, 120 Aden, Eclipse of the Sun, 175 Esthetic Defects of Photographs, 21 Albert, 243 Albert-type, 244 Albumen Pictures (see White of Egg) 32 Aniline Press, 246 Art and Photography, 277 Ai'tificial Light and Photography, 68 ^ Asphalt, 9 Astronomical Photography, 171 Bank-notes copied by Photogra- phy, 11 Barometrical Observations by Pho- tography, 200 Beoquerel, 273 Berkowsky, 174 Brewster, 99 Bromine, 109 Buildings, control of, by Photo- graphy, 220 Burchard, 254 Cabinet-shape, 53 Camera, 7, 89 Carbonio'*'Acid, Decomposition of Light by, 76 Cartes de Visite, 53 Charcoal Pictures, 239 Chemical Ink, 5 China Decorations, Photographic, 213, 257 Chloride of Silver, 4, 110 Chromic Oxide, 222 Chromic Photography, 221 Chromium, 222 Collodion, 34 Contr asts of Light -and Shade re- proQuced in rnotography, 126 Copper-plate Printing, Photogra. phio, 10, 215, 226 Copper Salts in Photography, 267 Copying Frames 26 Corona, Photography of the, 185 Daguebee, 12 Daguerreotype, 13, 14 Davy, 5 Development, 17, 42, 113 Disderi, 36 Distance, 142 Draper, 17 Dubroni, 2C8 Eqg,' White of, 33 Electric Light, 71 Elements, 107 Encaustics, 206, 255 Ettinghausen, 16 286 INDEX. Facsimile, 241 Field Photography, 16G Fixi ng. 2 4 Fixing Natron, 24 Foreshortening in Perapective, 148 Fritzsch, 2Q9 GirpiEiD, 270 Galvano-plastio, 227 Genre, Tableaux de, 126 Glass, its Combination with Light, 269 Glass -Painting and Photography, 259 Globes, Abnormal rendering of, by Photography, 124 Greens, 209, 213 Groups, Pictures of, 153 Gun Cotton, 34 Hali-tones reproduced by Helio- graphy, 230; by Photo-litho- graphy, 252; in Photo-reliefs, 234 Heliography with Asphalt, 10; with Salts of Silver, with Salts of Chromium, 214 Heliopietor, 204 Herschel, 24 Homsilyer, 4 Images, Optical, by Eefraction, 83 Accuraoy of Photographic, 120 ^"^ Their dependence on the Artist, the Apparatus, and the Original ; their Differ- ence and Beality, 120-133 Infernal Stone, 5 Instruments, Treatment of Scien- tific, by Photography, 200 Iodine, 17, 109 Iodide and Bromide of Silver, 113 Iron, Salts of, Photography' with, 265 JOUBEKT, 257 Jurisprudence and Photography,217 KaestbN, 16 KeUner, 252 Kirchoff, 63 Landsc^jb and Photography, 169 Land- Surveying Photography, 166 Lantern, Magic, 93, 283 Leaf Prints, 26 Leisner, 258 Lenses, 87 Libraries, Use of Microscopic Pho- tographs in, 210 Light, as a Chemically-Operative Agent, 55 Its Nature, 56 ■Chemical Effects of, 103 -Electric, 71 -Lime, 70 -Magnesium, 69 -Sky, 77 -Sun, 75 -Eefraction of, 83 -Pictures, 22 -Pans Paper, 23 Lithography, 248 Magic Cigars, 214 Magic Photography, 215 Magnesium Light, 69 Magnifying Apparatus, 15S Medical Kesearch and Photogra- phy, 202 Mercury, 17, 112 Meydenbauer, 171 Micro-photographs, 208 Microscopes, 207 Microscopic Photographs, 209 Minerals, Change of, by Light, 270 Mitscherhoh, 122 Morse, 17 Moser, 16, 103 Nateon, Hypo-sulphite of, 44 Negatives, 28, 30 Negative Process, 37 Negative Eetouohe, 40 Neumeyer, 201 Nifepce Nicophore, 9 INDEX. 287 Nilpce de St. Victor, 33, 2174 Obbenbttbe, 245, 257, 268 Oidtmarm, 258 Osborne, 251 Ozone, Formation of, by Light, 108 Palino of Photographs, 55 Panoramic Apparatus, 165 Pantograph, 231 Paper, Durable, Sensitive, 30 Paper Negatives, 28, 30 Paper Photography, 23 Parallactic Exposition, 172 Patterns, Photographic, 220 Paus Process, 24 Pea-Sausage, 264 Perspective, and its Influence on Photography, 134 Perspective Foreshortenings, 135 Petzval, 19 Phosphorus, Sensitive to Light, 108 Photographs from Photographs, 158 Photography Development of, Mo- dem, 31 Photo-lithography, 248 Photo-reliefs, 230 Photo-zincography, 250 Photometer, 262 Physical Effects of Light, 1 Pigment Press, 239 Plates, Cleaning of, 39 Poitevin, 239, 243, 251, 257, 275 Ponton, 224 Porta, 8 Portraits, 154 Portrait Objectives, 19 Portrait Photography, 149 Choice of Dress for, 153 Influence of Weather on, 155 • Influence of the Size of the Picture in, 156 Positive, 29 Potash and Glue, 225 ■ Prisms, 62 Protuberances, 183 Pretsoh, 227 Protection of Photographs, 45 PyrogalUo Acid, 35 Pyro-photography with Salts of Silver, 212 Qtjioksilvee. (see Merourj') Eepraotion of Light, 83 Relief (High and Low) Printing, 235 Seliefs, 234 Belief Press, 237 Eetouche Negative, 49 Rutherford, 187, 190, 191 Sachse, 16 Sand-blowing Process, 260 Scamoni, 215, 230 Shades, 135 Sohonbein, 34 Schools, 277 Seebeck, 273 Sensibilisators, 115 Silver, Hypo-bromide of, 111 Hypo-chloride of, 111 Contents of Photographs, 119 109 Hypo-iodide of, 111 Salts, Effects of Light, on, Use of, in Photography, 119 Solar Camera, 95 Sun and Sun-spots, taking of the, 186 <- Sunlight, Chemical Effect of, 77 Reflected, 74 Clearness of, 75 Spectrum, 62 Spectral Analysis, 63 Spectral Lines, 63, 194 Steel Engraving, Photographic, 226 Standpoint, Effect of the, 146 Stone, 199 Stereoscope, American, 102 Stereoscopic Landscapes, 163 Stars, taHng of, 188 Strengthening, 47 288 INDEX. Talbot, Fox, 23, 223 Talbot-type, 23 Paper PKotograplis, 23 Talbot, E., 30 Tent Photography, 161 Tessie de Mothay, 243 Thermometers Registered by Pho- tography, 200 Tilghmaim, 260 Tones and Colours, 59 Tones of Paper Fiotxires, 52 Topaz, its Sensitiveness to Light, 271 Transparent Pictures on Glass, 163 TJndtjlatioxs of Light, 57 Uranium Salts, 267 Vaenisf, 4G Venus, Transit of, 197 Visite, Cartes de (see Cartes) Wax, 5 ■Warren de la Rue, 174, 194 Wedgwood, 5 Wheatstone, 98 White of Egg (see Egg) Willis, 247 Woodbury, 237 Teilow Light, Effect of, on Plants, 80 On Salts, 42, 63 Zenckee, 275 Zincography, 250 THE END. International Scientific Series, D. Appleton & Co. have the pleasure of announcing that they have made arrange- ments for publishing, and have recently commenced the issue of, a SifRiES of Popular Monographs, or small works, under the above title, which will embody the results of recent inquiry' in the most interesting departments of advancing science. The character and scope of this series will be best indicated by a reference to the names and subjects included in the subjoined list, from which it will be seen that the cooperation of the most distinguished professors in England, Germany, France, and the United States, has been secured, and negotiations are pending &r contributions from other eminent scientific writers. The works will be issued in New York, London, Paris, Leipsic, Milan, and St. Petersburg. The International Scientific Series is entirely an American project, and was originated and organized by Dr. E. L. Youmans, who spent the greater part of a year in Europe, arranging with authors and publishers. The forthcoming volumes are as follows : Prof. LoMMEL (University of Erlangen), Optics. (In press.) Rev. M. J. Berkeley, M. A,, F. L. S., and M. Cooke, M. A., LL. D., Fungi ! their Nature^ Influences^ and uses. (In press.) Prof. W. k-iNGDON Clifford, M. 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Prof. Leuckart (University of Leipsic), Outlines of A nimal Organization. Prof. LiEBREiCH (University of Berlin), Outlines of Toxicology. Prof. KuNDT (University of Strasburg), On Sound, Prof. Rees (University of Erlangen), On Parasitic Plants. Prof. Steinthal (University of Berlin), Outlines of the Science of Language. E. Alglave (Professor of Constitutional and Administrative Law at Douai, and of Political Economy at Lille), The Primitive Elements of Political Con- sHiutwns. P. Lorain (Professor of Medicme, Paris), Modern Epidemics, Prof. Sch&tzenberger (Director of the Chemical Laboratory at the SorbonneJ. On Fermentations. Mons. Debray, Precious Metalt. opinions of the Press on the " International Scientific Series.'' Tyndall's Forms of Water. I vol., i2mo. Cloth. Illustrated Price, $1.50. " In the volume now published. 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It not only well sustams the char- acter of the volumes with which it is associated, but its reproduction in European coun* tries will be an honor to American science," — New York Tribune. " All the chemists in the country will enjoy its perusal, and many will seize upon it as a thin^ longed for. For, to those advanced students who have kept well abreast of the chemical tide, it offers a calm philosophy. To those others, youngest of the class, who have emerged from the schools since new methods have prevailed, it presents a, generalization, drawing to its use all the data, the relations of which the newly-fledged fact-seeker may but dimly perceive without its aid. . . . To the old chemists, Prof. Cooke's treatise is like a message from beyond the mountain. They have heard of changes in the science; the clash of the battle of old and new theories has stirred them from afar. The tidings, too, had come that the old had given way ; and Httie more than this they knew. . . . Prof. Cooke's -New Chemistry' must do wide service in bringing to close sight the little known and the longed for. ... As a philosophy it is elemen* tary, but, as a book of science, ordinary readers will find it sufficiently advanced."—* Utica Morning Herald, D. APPLETON & CO., Publishers, 549 & 551 Broadway, N. Y- opinions of the Press on the '''•International Scientific Series,^* VII. , The Conservation of Energy. By BALFOUR STEWART, LL. D., F. R* S. With an Appendix treating of the Vital and Menial Applications of the DociHne. I vol., i2mo. Cloth. Price, $1.50. _ ** The author has succeeded in presenting the facts in a clear and satisfactory manner, using simple language and copious illustration in the presentation of facts and prin- ciples, confining himself, however, to the physical aspect of the subjecL In the Ap- pendix the operation of the principles in the spheres of life and mind is supplied by the essays of Professors Le Conte and Bain." — Ohio Farmer. " Prof. Stewart is one of the best known teachers in Owens College in Manchester. ** The volume of The International Scientific Series now before us is an ex- -cellent illustration "of the true method of teaching, and will well compare with Prof. Tyndall's charming little book in the same series on ' Forms of Water," with illustrar tions enough to make clear, but not to conceal his thoughts, in a style simple and brief. ''—Christian Register, Boston. " The writer has wonderful ability to compress much information into a few words. It Is a rich treat to read such a book as this, when there is so much beauty and force combined with such simplicity. — Eastern Press. VIII. Animal Locomotion; Or, WALKING, SWIMMING, AND FLYING. With a Dissertation on Aeronautics. By J. BELL PETTIGREW, M. D., F. R. S., F, R. S. E.^- F. R.C. P.E. I vol., i2mo Price, $1.75. *'This work is more than a contribution to the stock ofentertajning knowledge, though, if it only pleased^, that would be sufficient excuse for its publication] . But Dr. Pettigrew has given his time to these investigations with the ultimate purpose of solv- ing the difficult problem of Aeronautics. To this he devotes the last fifty pages of his book. Dr. Pettigrew is confident that man will yet conquer the domain of the air." — N. Y. yournal ^ Commerce. "Most persons claim to know how to walk, but few could explain the mechanical principles involved in this most ordinary transaction, and will be surprised that the movements of bipeds and quadrupeds, the darting and rushing motion of fish, and the erratic flight of the denizens of the air, are not only anologous, but can be reduced to similar formula. The work is profusely illustrated, and, without reference to the theory it is designed to expound, will be regarded as a valuable addition to natural history." '—Omaha Republic. D. APPLETON & CO., Publishers, 549 & 551 Broadway, N. Y, opinions of the Press on the ^^International Scientific Series" IX. Responsibility in Mental Disease. By HENRY MAUDSLEY, M. D., Fello'.v of the Royal College of Physicians ; Professor of Medical Jurisprudence in University College, London. I vol., i2mo. Cloth. . . Price, $1.50. *' Having lectured in a medical college on Mental Disease, this book has been a feast to us. It handles a great subject in a masterly manner, and, in our judgment, the positions taken by the author are correct and well sustained." — Pastor and People. ** The author is at home in his subject, and presents his views in an almost singu- larly clear and satisfactory manner. . . . The volume is a valuable contribution to one of the most difficult, and at the same time one of the most important subjects of inves- tigation at the present day. " — N. V. Observer. *' It is a work profound and searching, and abounds in wisdom." — Piitshtirg Com- tnercial. *' Handles the important topic with masterly power, and its suggestions are prac- tical and of great valje." — Providence Press. X. The Science of Law. By SHELDON AMOS, M. A., Professor of Jurisprudence in University College, London; author of "A Systematic View of the Science of Jurisprudence," *' An English Code, its Difficulties and the Modes of overcoming them," etc., etc. I vol., i2mo. Cloth Price, $1.75. *^ The valuable series of * International Scientific ' works, prepared by eminent spe- cialists, with the intention of popularizing information in their several branches of knowledge, has received a good accession in this compact and thoughtful volume. It is a difficult task to give the outlines of a complete theory of law in a portable volume, which he who runs may read, and probably Professor Amos himself would be the last to claim that he has perfectly succeeded in doing this. But he has certainly done much to clear the science of law from the technical obscurities which darken it to minds which have had no legal training, and to make clear to his May* readers in how true and high a sense it can assert its right to be considered a science, and not a mere practice." — Th6 Christian Register. "The works of Bentham and Austin are abstruse and philosophical, and Maine's require hard study and a certain amount of special training. The writers also pursue different lines of investigation, and can only be regarded as comprehensive in the de- partments they confined themselves to. It was left to Amos to gather up the result and present the science in its fullness. The unquestionable merits of this, his last book, are, that it contains a complete treatment of a subject which has hitherto been handled by specialists, and it opens up that subject to every inquiring mind. . . . To do justice to ' The Science of Law' would require a longer review than we have space for. "We have read no more interesting and instructive book for some time. , Its themes concern every one who renders obedience to laws, and who would have those laws the best possible. The tide of legal reform which set in fifty years ago has to sweep yet higher if the flaws in our jurisprudence are to be removed. The process of change cannot be better guided than by a well-informed public mind, and Prof. Amos has done great service in materially helping to promote this end," — Buffalo Courier. D. APPLETON & CO., Publishers, 549 & 551 Broadway, N. Y. opinions of the Press on the '^International Scientific Series." XI. Animal Mechanism, A Treatise on Terrestrial and Aerial Locomotion. By E. J. MAREY, Professor at the College of France, and Member of the Academy of Medicine. With 117 Illustrations, drawn and engraved under the direction of the author. I vol., i2mo. Cloth. .... Price, $1-75 " We hope that, in the short glance which we have taken of some of the most im- portant points discussed in the work before us, we have succeeded in interesting our readers sufficiently in its contents to make them curious to learn more of its subject- matter. We cordially recommend it to their attention. *' The author of the present work, it is well known, stands at the -head of those physiologists who have investigated the mechanism of animal dynamics — indeed, we may almost say that he has made the subject his own. By the originality of his con- ceptions, the ingenuity of his constructipns, the skill of his analysis, and the persever- ance of his investigations, he has surpassed all others in the power of unveiling the complex and intricate movements of animated beings." — Popular Science Monthly. XII. History of the Conflict between Rehgion and Science. By JOHN WILLIAM DRAPER, M. D., LL.D., Author of " The Intellectual Development of Europe." I vol., i2mo. Price, $1.75. "This little ' History' would have been a valuable contribution to literature at any iime, and is, in fact, an admirable text-book upon a subject that is at present engross- ing the attention of a large number of the most serious-minded people, and it is no small compliment to the sagacity of its distinguished author that he has so well gauged the requirements of the times, and so adequately met them by the preparation of this volume. It remains to be added that, while the writer has flinched from no responsi- bility in his statements, and has written with entire fidelity to the demands of truth and justice, there is not a word in his book that can give offense to candid and fair- minded readers."— A''. K ^WBr'M^-Z'oj^. . , , " The key-note to this volume is found in the antagonism between the progressive tendencies of the human mind and the pretensions of ecclesiastical authority, as devel- oped in the history of modern science. No previous writer has treated the subject from this point of view, and the present monograph will be found to possess no less originality of conception than vigor of reasoning and wealth of erudition. . . . The method of Dr. Draper, in his treatment of the various questions that come up for dis- cussion, is marked by singular impartiality as well as consummate ability. Through- out his work he maintains the position of an historian, not of an advocate. His tone is tranquil and serene, as becomes the search after truth, with no trace of the impassioned ardor of controversy. He endeavors so far to identify hunself with the contending parties as to gain a clear comprehension of their motives, but, at the same time, he submits their actions to the tests of a cool and impartial examination."— A'. Y. Tnhme. D. APPLETON & CO., Publishers, 549 & SSi Broadway, N. Y. Recent Pub lication s.— scientific, THE PRINCIPLES OP ]»LENXAL PHYSIOLOGY. With their Ap. plications to the Training and Discipline of the Mind, and the Study of its Morbid Conditions. By W. B. Carpenter, F. R. S., etc. Illustrated, zzmo. 737 pages. Price, $3.00. " Tbe work is probaMy the ablest ezposition of the subject which has beeo given to the world, and goea far to establish a new system of Mental Pbilosopby, upon a much broader and more substantial oasis than it has heretofore stood." — St. Louis Democrat- " Let us add that nothing we have said, or in any limited space could say, would ^ve an adequate con- ception of the valuable and curious collection of £ict« bearing on morbid montal conditions, the learned physiological exposition, and the treasure-house of useful hinte for mental training, which make this large stiA vet very amusing, as well as instructive book, an encyclopsedia of well-clussified and often very startling psychological experiences." — London Spectaior. THE EXPANSE OP HEAVEN. A Series of Essays on the Wonders of the Firmament. By R. A. Proctor, B. A. " A very charming work ; cannot fail to lift the reader's mind up ' through Nature's work to Nature's God.' " — London Standard. " Fro£ K. A. Proctor is one of the very few rhetorical scientists who have the art of making science popular without m^dng it or themselves contemptible. It will be hard to find anywhere else so much skill In effective expression, combined with bo much genuine astroaomical leamijig, as ia to be seen in his new volume." — Christian Union. PHYSIOLOGY FOR PRACTICAL USE. By various Writere. Edited by James Hinton. With 50 Illustrations, i vol., izmo. Price, $2.25. " This book !a one of rare value, and will prove useful to a large class in the community. Ita chief recommendation ia in it« applying the laws of the science of physiology to cases of the deranged or diseased operations of the organs or processes of the human system. It is aa thoroughly practical as is a book of formnlaa of mediclnej and the style in which the information is given is so entirely devoid of the mystification of technical or scientific terms that the most simple can easily comprehend it.''— £on^ Journal. . ,„„.„., " The initial number is admirably constituted. —-fiWOTm^-ilTar/. _^ ' In our opinion, the right idea has been happily hit in the plan of this new monthly. .ffalo Courier. .. , j ■ • ' A journal which promises to be of eminent value to the cause of popular education m this country." — N. V. Tribune, IMPORTANT TO CLUBS. The Popular Science Monthly will be supplied at reduced rates with any periodi- cal pubUshed in this country. ' „ • Any person remitting Twenty Dollars for four yearly subscriptions will receive an ex- ba copy gratis, or five yearly subscriptions for $20. The Popular Science Monthly and Appletons' Journal (weekly), pet annum, f 8.00 1^' Payment^ in alt cases, must he in advance. ■« tv 1. 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